I do AQA so I don't know how much it differs, but some general tips would be to make sure you understand the aim/message of the poems. I think that's the hardest part of unseen poetry. Read the poems until you understand what they are saying, then it's easier to compare how they present a theme. Find points on how the messages differ, and in your analysis make sure to cover a range of poetic techniques regarding how they link to the way they present a theme. This includes the tone, metre, rhythm, form, as well as language techniques. The gender of the poet may also be significant. With structuring the essay, I would start each paragraph with a comparative topic sentence between the poems, before writing about one of the poems, then the next paragraph readdresses the comparative point and writes about the other poem.
Again, don't know how much of this applies to your exam board, but hope it helps a bit