Developing those skills takes time: none of this is a trick that can be done in one lesson. You need to think in terms of multiple lessons, and really over multiple years. Ideally, this starts in the first year of secondary school and only stops in the last year of it: teachers should think in terms of a Literature line running through the years and levels, and not just in terms of individual lessons. Critical thinking skills are complex cognitive processes which you cannot hope to address in one day, or even one year: it needs to be built progressively, and you should not expect to be able to measure it at every stage.
Don't expect to be able to measure everything, and certainly not after each lesson: thinking, considering multiple perspectives, not having a simple, clear and definite answer, asking questions, asking more nuanced questions...those things are hard, sometimes scary, and destabilising for many teachers and most learners. It all takes time to build up, flesh out and strengthen, and constant school evaluation of that process will not only do nothing for you, it's also likely to demotivate everyone. It will also contradict what you set out to do: expand minds.
This is not to say that evaluation is impossible, and the reality of schools is that testing is an integral part of their functioning (whether they like it or not). But do consider that evaluation is a tool with certain goals (mass assessment, determination of abstract levels, administration of large numbers, external framework-based demand) which may not suit your own aims.
Do consider also that putting numbers on something doesn't necessarily reveal any truth about it, or, if it does, that this truth might only be a reflection of the tool used to find it - a circular problem if there is one.
Do consider, finally, how long it took any of us to become open-minded, critical thinkers, reflective professionals with a certain ability to interpret the world - it took us many, many years, and the process is never finished: we are still all learning to do all of that well. In the context of a school, where pupils have multiple subjects, large numbers of tests and a great deal of very attractive options besides school, developing strong critical skills will necessarily take time.
You do not always need the full lesson to work on those skills. It is perfectly possible to develop those critical thinking skills through small-scale, short activities as well. Too often, literature lessons are seen as needed the full class-time, yet you can work on developing those skills in small bursts as well. For example, showing a particular picture, advert, painting and have everyone 'read it' (i.e. analyse, decode, interpret) can take 15 minutes; doing a critical thinking skill activity around a dilemma or moral situation can be done for a part of the lesson. Those skills need time to develop, none of this is a trick: do, repeat, repeat again, and again, and again...
Make connections between texts: using a text once and then never referring to it anymore is a waste. Each text, each author, will have their take on a theme. So once you have discussed a text, do not forget about it for the rest of the year: refer to it when discussing a new text, for example by asking how the same topic and/or theme (or related theme) is handled in one and the other text; which nuances are introduced to modify our first discussion; how this next text completes our understanding of the issues raised with the first text. To interpret means to use everything you know to make sense of something - part of that 'everything' must include previous discussions from previously read texts.
It's also important to remember that literature is a continuum, a game of call-and-response (or action-reaction) through time, as Gombrich (1950) put it: 'Each generation is at some point in revolt against the standards of their fathers; each work of art is related by imitation or contradiction to what has gone on before' . Texts, authors, movements, do not emerge out of nothing - the Romantics did not appear one day out of the blue, nor did the Angry Young Men: they were reacting to the Art of their forebears, adding to it, substracting from it, changing it, transforming it, bringing new ideas and, more importantly for us, new arguments for old themes. Literary periods are an academic super-imposition on the natural flow of time and creation, and we should not confuse the practicality of dividing literature into periods with the reality of literature itself.
Have a topic recur with different themes: you may be happy to go to war and looking forward to fighting (or defending your country, or protecting your family or your values), but you might also be very afraid of doing so, or you might refuse to do so, or you might go but find it morally distasteful. There are many ways (= interpretations) to approach a topic ('War' in this case): multiplying the angles to deal with a topic will increase the complexity of thinking and deepen your reader's understanding of an issue. This is obviously related to the point above: make connection between texts. Ultimately, you will benefit from thinking in terms of arguments on a theme, so that you may have one topic, linked to different themes, all based on various arguments.
Identify your goals clearly: things won't happen if you don't - make a difference between, for example, teaching Literary history, Literary techniques, Literary analysis...and interpreting. It's no good going into the lesson thinking vaguely about 'Doing literature': literature is no different from grammar, or English skills: you need to know what, why, what for and how. Be precise, be prepared, know what you want to achieve (interpreting, discussing, thinking) but be flexible about how to get there.
Once you are clear about what you think, what is possible, what is desirable, and the way to get there, you can separate out what creates full engagement with texts (the approach on this site, i.e. the text as a means to an end) and what sees the text as an end in itself (e.g. text as cultural artefact, text as canon, text as example of a movement, text as historically significant, text as exercise for terminology)
Prepare, prepare, prepare: know the text inside out, have multiple possible themes/interpretations so that you will be open to any new idea from the learners. Don't just read your material once: that will lead you to think there's only one 'meaning' and you might then not be open to any other interpretation. Read your text(s) multiple times, pen in hand, annotate, amass material to think from (observe), be ready. It's much better to know a few texts inside out than know superficially a large amount of texts. That type of preparation is also crucial for asking questions and leading the discussion: see the section on note-taking and annotating.
You must be prepared to rebound on left-field remarks as much as naive ones, and to make use of any consideration, however far-fetched they may seem to you, put forward by readers. You must also be able to link up those different remarks and interpretations to the text - and if need be, question them on the basis of the text, or complement them with bits from the text. In other words, you must own the text, and not let it be external to you, your guts, your brain and your heart.
Know how (and when) to let go of your plans to make the most of what is said by learners: this is difficult for teachers, and difficult for learners too. Learners will want one answer ('but teacher: what does it mean then?') as that's what school has got them used to. Teachers will often find the 'one answer' approach very comforting as well (there's a right, there's a wrong, it makes it all easier). Resist those temptations, be ready to let go and follow your learners' answers where they will lead you.
When you prepare, you will end up with different themes, arguments and quotes that could be discussed, and questions you'd like to ask. But if you want to achieve interaction, you may have to ditch your plans after the first question if the discussion takes off right then. Do not interrupt a discussion for the sake of your lesson-plan, and remember your goal: to have learners think, ponder and discuss - that is the aim. If many of the questions you prepared are never asked, if some of the themes you identified don't get discussed, but if the discussion takes off in ways you had not anticipated: that is the price to pay...and it is a good price to pay.
So you should give free rein to the ideas, and be prepared to let go of your preparation. Remember that your preparation is not designed to lead to a full analysis of the text: it is designed to make you the owner of the text and the leader of the discussion. Letting go is scary, yes, it's filled with uncertainties, true...but it's the best feeling ever when it works!
The teacher's role is central: not in giving answers but in eliciting ideas, interpretations, depth, justification, and to widen their scope. You must consider the added value of putting your learners in groups to discuss a text they don't understand and have little idea about: are you doing this for interpretive reasons, or because it's good for them to speak English for a while? Consider what they may be able to do on their own, compared to what you could lead them to do (and think, and see) if you take a more central role? As such, you will ask questions, ask follow-up questions, and enable the discussion to continue, take new directions and suggest new ideas. You never impose a view or an interpretation, you never curtail learner's interpretations by saying 'no, that's not good', or 'no, that's not what the text is about', but you do lead: you enable the taking of freedom.
That is where your preparation will come into its own: because you will have identified themes and arguments beforehand, you will be in a position to ask pertinent, probing questions when you need to; you will be able to suggest a new line of enquiry if the discussion dies down; you will be able to present alternative readings and interpretations and have your learners react and respond. You're not a coach, or an observer: you are a central participant in this process.
Choose texts that serve your purposes, irrespective of their genre, status, age, canon or literary pretentions. Ask yourself a simple question: is that text 'teachable'? Does it give me (the teacher) enough material to work with in class? Are there thematic questions I could come up with, are there nuances that would be the basis for discussions, are there moral ambiguities, dilemmas, that could be discussed in class?
And of course, it pays to consider the nature of the text in terms of readability, accessibility, pace, narrative techniques (voice: who speaks; structure; tone) and genre. Most inexperienced readers will struggle with very descriptive texts, slow-paced ones, heavily psychological ones, absence of dialogues, absence of strong events. Further, different backgrounds, reading experiences and tastes will make it almost impossible to wax lyrical about stylistic values or appeal to a response based on 'literary qualities'.
Your purpose, not your taste, is the point here. I'm happy using a story or a novel that I may not read for myself or think much of in pure literary terms if that text lends itself to discussions. 'The Outsiders', that classic American school-text, is objectively a poor novel in terms of structure, characterisation and representation of the world. But in many ways, these real weaknesses are also perfect starting points for many discussions. As a reader, I sometimes had to laugh at the incongruities and flaws in that novel, but as a teacher I can see beyond those things because I consider my purpose, and not my own tastes. If I can find a text that marries those two, all the better - and I would never recommend using a text you truly dislike. But I do not dislike 'The Outsiders': it's both a very poor novel, and an excellent one - it all depends on your goals.
Choose texts that learners can handle, but don't sacrifice your principles to make it 'fun'. Asking someone who loves Harry Potter to read it again, or giving a text 'for enjoyment' may be fine, but it remains important to consider what it actually means. 'Having fun' while reading may indeed lead to more engagement, but it might equally well lead to skimming the surface by attending to the story and the plot(s), neglecting to reflect on the text any further. 'Having fun' often equates with 'Escapism' for inexperienced readers, which quickly leads to remarks like 'I'm reading this for fun, I'm not trying to think about it too deeply', or 'What I read for fun is not worth reflecting on really - it's not literature you know!'. Remember that 'fun' can also mean thinking new thoughts, having new insights, participating in good discussions, arguing your point of view, discovering things and people and ideas - using your brain is fun, in other words.
Similarly, do consider what is gained by having someone re-read a book they liked, for example Harry Potter: they might get great pleasure out of it, but how does that, in itself, either lead to thoughts or to reading other books? Such an approach seems close to a moral argument about reading, something like: 'As long as they're reading, it's all good, I don't mind what it is'. But such a view remains essentially a moral one, and it's hard to square that with your goals as a teacher as we describe them here. There is, objectively speaking, little reason why reading would be a morally good act - such a view is historical, and cultural, and promoted by those who read. Your goal is not a moral one, but a practical one: to introduce new ideas, new ways of thinking, of seeing, of reading, to educate, to broaden the horizons (all things which, objectively, are...morally good!)
Move away from Reader's Response: literature is great at showing different worlds and ways of thinking, and reader's response focuses on personal feelings only - it doesn't help learners consider other ways of seeing, feeling, thinking. It's possibly a good way to start a lesson, but it should not be the end-goal. Reader's response has a tendency to limit the text to an emotional, very personal and unreasoned take on a text, typically tapping into moral views (often unconscious ones, too). Hence readers' remarks like 'I don't like this book because the main character is really boring', or 'I don't like this book because I don't find the subject interesting'. Literature is a window on other worlds, views, feelings, ideas and perspectives, so it's really quite a waste to limit its scope to a personal response only. If you do start with reader's response, do make sure that you confront those responses to elicit understanding of differences, to identify cultural and idiosyncratic ways to think - we have an example here.
Do not try to please everyone by choosing consensual texts: open your learners' eyes and minds with something new to them. The world is vast, and texts reflect that. Your learners are diverse and varied, and it is unlikely you'll find one text, one author, one genre, that will please every single one of them. Much better to ask again: what do I want to get out of this from my learners? What is a good text to get them to think, to look around, to discuss?
Taste and Judgement are two different things. W.H. Auden, the English poet, said it well (1970): 'As readers, we remain in the nursery stage so long as we cannot distinguish between taste and judgement, so long, that is, as the only possible verdicts we can pass on a book are two: this I like; this I don't like'. Auden goes on to say that adult reading (in the sense of mature reading, not age in itself) distinguishes between taste and judgement - it is possible to dislike a text while recognising it is good, and offer possibilities; it is possible to like a text while seeing it is not very good. Do not be afraid to remind yourself and others of that distinction: explaining why one dislikes a text is difficult but very useful; choosing texts for what they can lead to is more important than choosing them based on your own tastes.
Use your lessons to expose learners to what they have never seen, thought about or experienced; don't keep using the same texts they're likely to have read (or liked). That only comforts them in what they already know or feel, whereas your objective is to make them reflect, not accept blindly or reproduce the same old experience. Reading new things can be scary, and trying new things too - but aren't teachers one of the way to discover new things? If your child loves bananas and you know they eat it with pleasure, do you then only feed them bananas? Don't you try strawberries, or pineapple, or pears? Don't you branch out and try cucumber, carrots or turnips? It is only by trying those new foods that your child will find out what they like and dislike, and why - and the world of books (and Art in general) is no different.
There is space enough for all of us: we do not need to think in absolutes, and there is no reason to claim that this method is the best, or that way is the ultimate key to success. There's also no reason to claim that one method fits all, or replaces all, or does it all. I drink coffee in the morning and some wine in the evening - rarely the other way around. I do not eat meat in sauce for breakfast, nor do I eat cereals for dinner. Yet those beverages and meals are not mutually exclusive: they coexist without difficulty since each has a raison d'être. They are not opposed to one another, they are complementary. Our approach to literature in class is similar: it does not exclude others, or claim to replace them - it can (and, in our view, must) exist alongside other types of lessons.