How should I write a critical analysis in English?

To write an essay that critically analyses a piece, it is crucial to discuss the passage in ways that are of interest to YOU. Approaching the essay from a specific angle is a good way to start, and you may, if you wish, add a line of argument to give it greater structure and coherence.

Begin by alluding to a couple of key things that you notice about the passage which particularly grab your interest. Introduce them as topics to be opened up for discussion, which you will then flesh out in the main body of the essay by means of examining technical features. 

You may wish to consider things such as genre/mode, vocabulary, the author's approach to imagery, tenses, form, rhyme, narrative structure, the title of the piece, the length of syntactical units, the author's use of punctuation, and the relation between form and content. It is also crucial that you consider the relevance of technical features to your interpretation of the piece, as well as considering how succesful it is at what it is setting out to achieve.

Most importantly, examiners are looking for flair and individuality, so don't be afraid to approach things from unusual angles, and to be imaginative and even figurative in your use of language!