How should I structure an essay on an unseen prose extract?

You should begin by reading the extract a couple of times. Afterwards you should read it through again but this time you could attempt to annotate the extract whilst thinking about the form, structure and language of the extract. This will include considering the genre, how the structure of the paragraphs and syntax influence the meaning of the extract and finally the meaning behind the use of language. You can then use this information to form an overall argument in response to the specific question. 
In your introduction you should briefly lay out the basic nature of the extract, it's genre, and perhaps its literary historical context. You should then introduce your overarching argument briefly and set out the central points you will use to make this argument. Then structure the bulk of your essay around these points. It's important to have a clear structure for this part of your essay and therefore it is probably best to create distinct paragraphs for each point. It's also important to to conclude each paragraph with a clear statement as to why this point was relevant to answering the question. Finally, to conclude your essay you should surmise these points and their relevance to reiterate the argument you set out at the beginning.