How do you approach writing a unseen poetry commentary?

The first thing you should do when you see a poem is read it and write down first impressions. This should include what on first glace you think the poem is about. It is highly likely, that your first impression is incorrect, but first impressions matter as your first perception of the poem in terms of mood is an important component that you will write about when analysing the piece. Is the poem in free verse, blank verse, or can you easily see that it is a sonnet or elegy? Read the poem through another two times: what comes to mind? Write down any words that come to mind and think of all possible synonyms. Now that you have hopefully gotten a vague understanding of the purpose and intent of the author, work through the poem stanza by stanza line by line. The poem is probably progresses either chronologically or by themes, are these clearly devided by stanzas or is the structure unusual? Having read the poem about five - six times, you should have an idea of three or four main points you want to make, now it is time to look for evidence.  Line by line, focus on any literary devices (metaphors? similies? personification? imagery?). Finally step back and visually analyse the poem, does is visibly seem to follow a certain structure? Is there rhyme? 

Now that you have 'analysed' the poem, it is time to clearly express your findings in essay form. Hopefully while looking for literary devices, you wrote down in categories main features, so your goal now is to formulate a clear thesis and unfold your argument.