Most importantly, don’t rush! Take time to reread the poems with the theme of the question in mind.
Make sure you understand everything - plan the essay first. You might find it helpful to draw out a two column table, with a column for each poem, and write down parts of the poems that address the theme of the question. It might sound obvious, but think about them as poems (look closely at, for example, verse form, structure, stanza, rhyme, sound, imagery, metaphor, vocabulary, tone). Think about what the poets are trying to convey - what is the effect of a certain metaphor or a certain rhyme scheme?
Clarity is important - your introduction should address the question, and set out how you are going to compare them. This will help you and the person reading it. In your plan, establish three central points. Each point should refer to both poems: remember that it is a comparison. Once you have established your three points, provide evidence from both poems, and then back them up.