How do I structure a commentary?

This varies from person to person. Some teachers recommend structuring your paragraphs according to literary devices, for example focusing on tone in one paragraph, structure on another, and figurative language on another. However, what worked for me and got me the 7 was to analyse it line by line, beginning a new paragraph whenever the poem has a clear shift (which could be seen in a new stanza or, in a one stanza poem for example, a sudden change in tone) as it ensured I discussed all the literary devices used in a coherent order that followed the poem's message.