Understand the critical lens and what it is asking of you. A critical lens is a certain viewpoint you should look through as you analyze the piece of literature you were assigned. It could be a school of thought, such as a "Marxist" or "feminist" critical lens, or it could be a quote expressing some sort of general opinion. Make sure you know what this critical lens is and what it's all about. Talk to your teacher or do additional research if necessary.
Analyze the piece of literature. The best thing is to reread it, if possible, with your critical lens in mind. As you read, highlight passages that seem to support the critical lens. For example, if you are supposed to look at the text through a psychoanalytic critical lens, passages that have to deal with a father-son relationship could be used to expound on an overall Oedipal theme in your paper.
Craft a thesis statement – an overview of what your essay is going to be about, containing the basic argument you will be making in your essay. It should create an argument about an aspect of the critical lens that can be supported by direct quotes from your piece of literature. If you were given a prompt as part of your assignment, make sure your thesis statement directly answers it.
Write the introductory paragraph, which should basically be an extension of your thesis statement. Use it to explain any concepts you feel you need to go into detail about, or to bring in anything you feel is necessary before you start getting deeper into the text.
Write your body paragraphs. You can go about this several ways: you can write each paragraph about a separate point you have to make about the text, or you can explain one point in a paragraph and then use the next several paragraphs to explain and analyze different passages that support it, then start a new point and do the same thing. The basic purpose of your body paragraphs is to go deep into the text and analyze it from the perspective of your critical lens. The topics of your body paragraphs must stem directly from your thesis statement.
Write your conclusion and tie everything together. You have several ways to go about it, but make sure this conclusion has a closing feel to it, and maybe adds a little something new, like a further explanation of the critical lens, or another analytical point that stems from earlier ones, so that it's not just recycling what you've said before.