Understand that the only requirement of a conceit poem is the inclusion of a prolonged metaphor. Length, rhyming mechanisms and poem structure can all vary dramatically.
Read a couple conceit poems to gain a better idea of the form. Classic examples of conceit poetry include William Shakespeare's Sonnet 18 ("Shall I compare thee to a summer's day") and John Donne's "The Flea."
Think of an interesting metaphor or simile that compares two completely different things. Examples include, "You are as beautiful as a sunset," "My fists are tornadoes" or "My body is a cave."
Use your metaphor as the beginning line in your conceit poem. Then find ways to prolong the comparison. For example, if you were comparing fists to tornadoes, you could write, "My fists are tornadoes. They crush spirits and crash through steel."
Continue the metaphor for as long as you can. If you want, add a rhyming mechanism to give your poem a nice rhythm and lyrical flow.