Choose a topic for your poem that consists of two things you would like to compare. One must be literal and one must be figurative. For the best results, choose broad topics or items that have many traits in common.
Make a list of the traits that the literal item and figurative item have in common. The longer and more detailed your list, the better because you will have many topics to choose from when you write. Expand upon the list by generating banks of words related to the common traits that you can incorporate into the poem.
Write the poem using the traits of the figurative item to describe the literal item. Continue making comparisons between the literal item and the figurative item without using the simile words, "like" and "as." Instead, write as though the literal item is the figurative item.
Read your poem several times to verify that the extended metaphor is clear and solid throughout the poem.
Ask a friend, family member or teacher to read your poem, and do not tell them what the metaphor is. If they can understand it, you know that your metaphor is working well.
Revise your poem based on the feedback you receive so that the extended metaphor is clear.