A literary critical analysis includes a discussion of structure, thesis, language particulars and diction, plot and conflict, characters, and other literary devices of the literary element and literary technique categories, including mood and tone, tropes and conventions. In order to amass this information, you need to pay close attention to the elements and techniques, points and objectives in the writing as you come across them, paying special attention to subtle (or bold) changes in tone and mood as well as to words, phrases, or sentences that make, prove, or emphasize the author's thesis (major point).
To write your critical analysis, after your Introduction, you'll have a brief description or perhaps summary of the work you're analyzing followed a discussion of structural points. If you're analyzing a poem, this would include things like rhyme scheme, stanza construction, genre, and meter. If a book, this would include things like narrator, point of view, chronological orientation, overall mood (mood can change), and overall tone (tone can change). You then discuss literary elements and techniques, authorial style, the treatment of the work's thesis, and the work's effectiveness as these are relevant to your particular thesis statement--the point you wish to make about the literary work.
This may seem like a lot--and it can be--but it can also be abbreviated to suit the needs of your essay. For example, if my thesis is that a work is not effective because of faulty language, I may address structural elements as briefly as follows if I wish--if it serves my purpose to do so: The first person narrator establishes a despondent tone that matches the gloomy mood (same as atmosphere) as s/he paces back and forth in time between flashbacks, present day events, and anticipated future events while telling of the tragedy s/he witnessed on the slow boat to China two decades earlier.