Start by reading the unseen passages - it is fine to have a preference between prose and poetry, but never rule out either until you've had a look at both! Pick the passage which you have a deeper understanding of and that lends itself to an argument; your literary commentary must have a good structure and direction, so picking a text that allows you to explore with reference to specific themes/strands is good. Once decided, choose 3 (or even 4) strands that you would like to focus on in your commentary. Each strand must be unique and work well with quotations you will take from the text. Colour code these three strands and re-read the passage again, this time highlighting in accordance with the codes you have set up. This will allow you to focus on the parts of the text that back up different parts of your argument. Your commentary must have a brief introduction, body and conclusion to give structure to your essay, and the conclusion should contain a final illuminating piece of information for the examiner. Your introduction contain a form of thesis statement, outlining for the examiner what exactly you will be exploring to guide them. Once you have written this intro, you are ready to dive in!