A metaphor is comparison between two dissimilar things. To write a metaphorical poem, you can create your metaphor in a couple of different ways. You could use many different metaphors to compare one thing with many other things. Or you could have one long metaphor throughout your poem, which you expand on as you go. This is known as an extended metaphor. Either way you do it, a strong metaphor is a wonderful jumping off point for creative poetry.
Writing the Poem
Brainstorm! You need to develop your metaphor in order to write your poem. What do you want to compare? Do you want an extended metaphor that flows throughout your poem or would you like a steady theme that can incorporate many metaphors into it? The choice is yours, but you can figure out the best way to go by making a list of possible metaphors you could include. Remember, the more unique the comparison is, the more interesting your poem will be.
Put pen to paper. After you have a general idea of where you want your poem to go, just start writing. At this stage, you can't go wrong as long as you don't stop writing. If you had planned on using one metaphor throughout but find that more seem to occur as you write, go with it. Take advantage of your own natural creativity.
Edit and rewrite. Reread what you've done and decide what you like or dislike. Make sure your metaphor is not so subtle that no one will be able to find it or understand it. You also do not want it so direct that your reader has nothing to think about. Poetry is often a fine balancing act in this way.
When you're happy with your poem, see if others can pull your desired meaning out of the poem. Have someone else read your poem. Could the reader find the comparison you were making with your metaphor? Find out what effect your words have on the reader and decide if you still need to tweak it or not. Sometimes you may not have written what you first intended to, but that's okay as long as you're happy with the final product.