Find a poem that contains metaphor. "Happiness is a Riverbank" by Pauline Oliver is one example. Read the poem carefully and identify the poet's use of metaphor. Give your explanation of what is being compared and the purpose of the metaphor in the poem. The use of metaphor in this poem compares the peace and solitude of a riverbank with the peace and solitude that brings happiness. Think about how you might change this literary technique to suit what message you want your poem to carry.
Think about what you want to say and how you want to say it. Decide whether you want to turn your poem into one singular metaphor or a number of separate metaphors. Decide what you want your metaphor to represent. Perhaps you would like to talk about death by writing a poem about the coming of winter. You could also write a poem comparing the person you love to a rare flower.
Start writing. If you have decided to have a single metaphor running through the poem, including other metaphors at the same time is not a problem. Think about certain characteristics that your comparisons share and take advantage of this. Winter is cold and lonely, as death is. The season gradually changes, as the dying body does. These are similarities that will help make the metaphor in your poem clear and powerful.