Where do I even start in writing a commentary for unseen poetry?

Getting a page of poetry you’ve never seen before is daunting, and especially when you’re under time pressure, it’s easy to get stressed. For this very reason, it’s necessary to just take a few minutes to properly read the poem. Read it once, and I would even recommend that you read it again. From this, try to extract what exactly the poem is trying to say; if you had to condense its meaning into a single sentence, what is the poet actually talking about? Once you’ve established this, it’s time to move onto wider themes. Nature, religion, love and death are probably some of the most common examples, so if you’re really stuck, they are a good place to start looking.
Now that you know what the poet’s message is, and the wider concepts being used to convey it, you can move onto the language being employed by the poet. Alliteration, personification, metaphors, similes, hyperboles, synaesthesia, onomatopoeia, allusions, symbolism and oxymorons are just some examples of what you should keep an eye out for.
Finally, don’t forget about structure and syntax. Even if a poem doesn’t have an obvious rhyming scheme to talk about, there will almost definitely be some kind of structural organization or use of technique. In the case of the former, your work should be cut out for you: there might be rhyming couplets which are adding emphasis to just those lines, for example, or maybe the whole poem has a rhyme scheme. If it doesn’t have any rhyme, don’t panic - there are still many other structural points you can make. Enjambment is just one example, where there is a lack of punctuation and the line runs over into the next.
The most important point with all of these techniques is that you don’t just list them. It is important to know the key vocabulary and be able to identify them in use, but what will get you the marks is if you can explain why it is being used. Does it contribute to the message of the poem, or engage with the themes you identified earlier? Does it affect the tone of the speaker? Have a thesis in your essay. For example, “Poet X is concerned with the theme of death, which is apparent through his use of winter as the setting of the poem. This nature imagery serves to remind the reader of the inevitability of mortality and provide a sombre tone”.