There is no formula for poetry commentary. A poetic analysis can be written formally, dissecting the poem word by word. It can also be informal, focusing more on the experience of the author of the analysis and how the poem affected him. The important thing is to bring your reader some new understanding about a piece of poetry. If you can make him see something new, your analysis has proven worthwhile.
Say what you believe the poem is about. Many poems work at multiple levels, so you should try to relate the levels together. For example, the Robert Frost poem "The Road Not Taken" on one level is just about choosing which road to walk down during a stroll. On another level, however, the poem is often interpreted as a statement about personal choices and individualism. Both meanings are important.
Discuss the imagery in the poem. How do the images the poet uses contribute to the overall meaning of the poem? Is there anything surprising or captivating that jumped out at you while you were reading it, and why was it surprising? In a well-written poem, every word is carefully chosen, so don't be afraid to analyze the poet's word choice carefully.
Discuss the structure of the poem. Is it written as free verse, iambic pentameter, a limerick or some other poetic meter? How does the structure of the poem contribute to the meaning? For example, a war poem might have a marching beat to give a feeling of marching into battle. If you aren't sure what meter a peom has, see the list of poetic meters in Resources link.
Discuss the voice of the poem. What is the poet's attitude toward the subject of the poem, and does it change from the beginning to the end? How is the reader supposed to feel when reading the poem?
Write a conclusion. The conclusion is a good place to get more personal with your analysis. What did the poem mean to you personally? Did you enjoy it? Why or why not? How does this poem relate to your world and your life?