Think of your subject matter. Find a person or thing that you would like to write about. Make a list of everything that reminds you of them or parts of them, and split it into two columns. For example, in the first column you could put "Rebecca's eyes" and in the second "a blue sky." Be creative here and spend a long time thinking and jotting down ideas for comparison.
Decide on the form of your poem. You can be as strict or as loose as you want. If you would like a challenge, choose a sonnet where you have to write 14 lines in iambic pentameter and follow a specific rhyme scheme. Choose free verse if you would like more freedom, but this does not mean you can just write however you like. The poem has to follow some type of pattern.
Use descriptive imagery. Form pictures with your words and be detailed. This will help your reader visualize and understand your comparison.
Choose which figures of speech you would like to use. There are many options open to you when you want to compare one thing to another. Use simile and metaphor to link two ideas directly, and personification to write about an animal, plant or thing. You can also describe your subject by saying what it is not and using oxymoron or paradox to get your point across. Only speckle these elements throughout your poem because overuse will make it seem contrived.
Keep it short. A comparison poem cannot go on for hundreds and hundreds of lines or it becomes cumbersome. It will be more effective if you keep it to the point.