Write down several theme and story ideas you may want to tell in your poem. A theme can be anything from the dangers of nature to life and death. Once you have a theme in mind, you can begin constructing a story around that particular theme. Having a theme in mind can also help when it comes time to choose the words for your poem.
Structure your poem as you go. Not all poems need to have rhyming lines. Not all poems need to have specific structure in each stanza. If you want to confine yourself to a specific number of accented and unaccented syllables per line, you can, but don't force it. Allow the story to dictate the rhythm of the poem that best suits the telling of the story. Write in pencil and come back and erase if you need to.
Write a scene per stanza. Think of a stanza as a paragraph. Each of your stanzas, whether four lines long or eight, should encompass a particular scene in your story. You can have as many stanzas as you need to tell the story. Separate the stanzas in your poem by placing a blank line between them.
Choose words wisely. Unlike a novel, you don't have the space to go into all of the minute details. Every word in a poem has to mean something. Every line should move the story forward. You are striving for visual effect and story motion in as succinct a fashion as you can. Avoid words that readers will skip over (the, a, an, then, these). Go for concrete action words and those descriptive words that are not empty ("pretty" to describe something is empty). Take advantage of literary tools and figures of speech like analogies, similes and metaphors to get a point across.