Get a sense of what the poem is trying to say and how it makes you feel. Though not material for the paper itself, it is an important starting point. Think about what aspects of the poem interest you, what parts you don't understand or even what annoys you. You have a lot of latitude, so write about something you want to explore.
Choose the most focused thesis you can think of for the length of the paper. For example, you could limit discussion to the use of one symbol or type of imagery, a specific rhetorical device or one aspect of the narrator's perspective.
Read and re-read the poem to isolate all parts of it that apply to your thesis. In your notes, write down the line number and the reason you are making a note in the first place.
Think about how the imagery, tone, rhythm or perspective fit in with the overall picture of what the poem is trying to express. Successfully making this connection is what will get you a good grade.
Outline your paper, saving your most convincing points for last. As you structure your paper, you can organize it much like you thought about it, building up examples of the technical aspects to culminate in a greater understanding.
Write your paper, following whatever style guidelines your professor requires. If you've done your reading, thinking and outlining beforehand, this should be the easiest part of the process.
Get a smart friend to proofread your work. If you're working from a short poem, you might as well have her read that too. Sometimes, you can get so deeply involved in a poem that you overlook obvious aspects of it that might be important to include in your paper.