Choose a subject for which you want to find a poetic metaphor. It can be an action, a thing, or a person -- real or abstract. Suppose you're writing a poem about unrequited love, which is a common theme in poetry. You see a moon from your window, and you want to find a poetic metaphor for it to use in the poem.
Write down all the words that come to mind when you see the object -- either literally out of the window or with the mind's eye. In the example, it might be crescent, red, cold, silver, etc.
Determine the metaphorical concept. Suppose the writer feels that unrequited love is similar to losing a battle. Here, the metaphorical concept is a battle. Note that the writer could have selected another metaphorical concept. For example, love can also be thought of as a dance, a race or a child's play.
Reflect on other features of the metaphorical concept. What happens in a battle? There's a winner and a loser. Many people are wounded, and many die. There's usually a lot of blood and weapons. Get into the details.
Make the object and a feature of the metaphorical concept you chose identical to create a poetic metaphor from a metaphorical concept. For example, thinking of unrequited love, a poet might write the following line:
Ah, the moon --a silver bullet.