First I’d make some general points like…It is important to get a sense of what the piece is about before you do anything – always read an unseen twice if not three times so that you could summarise it in a couple of sentences to help ensure your commentary is focussed. What is the writer doing and why? Any bit of language you comment on should be linked to analysis. There’s no use pointing out that two words rhyme. Why do they rhyme?
Then I’d go into more specific things like approaching how to analyse a piece. By looking at the genre, the form – if there is an obvious form like a sonnet are there interesting deviations from that sonnet form worth commenting on? The tone and how the writer is creating that tone – is it through an ironic narrator – how is that sense of irony being created? Answering these three questions by picking three quotations you see as indicative of the piece could form the three paragraphs of an essay. Then we need to analyse exactly what is going on within the language, for example, the use of pronouns, adjectives, verbs (or lack of them) language tools; anaphora etc. but always linking these observations back to why the writer has chosen to use these tool in the way that they have.