Choose the character you want to write about. It could be a character you create or somebody famous. Study the character you're writing about and pick out the physical traits about him that are recognizable. These are traits you'll want to bring out in the poem.
Exaggerate details. Poems are shorter than stories. You have limited space to make your character come to life. Think caricature. You want your character to come off as larger than life.
Stretch the truth. Good character poems often do this. Try to put a humorous or fun spin on what really happened if you're writing a character poem based on a real person. This doesn't mean you can't stick with the facts if your intention is to write a serious poem, but if there's room for fun, modify the actual events by making them larger than life as well.
Don't worry about form. A character poem can be rhymed or unrhymed, even or uneven stanzas, or one long stanza. There are no hard and fast rules about form in character poems. The key is to define the character you choose clearly and describe your character's life or an event in his life in way way that entertains, informs, or enlightens anyone reading the poem.