Carefully read the work you are criticizing at least twice. During the first read, take notes of interesting or striking passages and note the pages on which they occur. Read over your notes to identify recurring themes before reading a second time, taking more detailed notes. Pay attention to oddities and things that stand out as well as major events, as they often lead to interesting observations.
Begin the writing process with a pre-write. This is a less formal version of your essay. You can try a number of different things to get started, such as imagining you are being interviewed by a friend, and just writing down how you would respond. Brainstorm and note whatever comes into your head about your topic. Do a drawing that diagrams the major events or themes as you see them.
Write an outline taken from your pre-write and notes to give you a framework for your essay. Filling in an outline is easier than trying to envision the essay as you go along.
Develop a thesis statement that is the foundation of your thoughts and what you will cover in the essay. This should be incorporated into the beginning of your essay.
Write your essay using your outline as a guide. Use the active voice. Start with a topic sentence for each paragraph, and flesh out the topic sentence in the paragraph while backing up your statements with examples from the work you are writing on.
Finish your essay with a recapitulation of your thesis statement, along with a concluding summary of your thoughts.