When it comes to unseen poetry there are many ways to approach the analysis. To begin with it is always best to read it, not once but twice, making sure to underline/note themes, imagery, symbolism and other points of interest.
After you kind of have a good idea of what the poet is trying to convey in the poem, a good technique to use is the P.E.E.
P-oint --> What is the point you're trying to make?
E-vidence --> What line(s) in the poem back up your point? Why?
E-xplain --> How? What effect(s) does it create?
Ultimately, in the exam situation you will obviously be under time constraints. However, fret not, utilising said system alongside an essay plan will help you construct a detailed analysis that will impress any examiner.