First, you need a subject for your poem. Something you want to write about.
EX. A tree
Then, try to feel what you're going to write about. Try to imagine that you ARE the thing you're writing about. Think about the plot, and how you want to describe what's going on.
EX. The trees brown, rough bark, the green leaves, the long. Spindly branches. Maybe it's fall, and the leaves are slowly changing color.
Lastly, just write what you feel. Let your emotions flow from your mind, through your pencil, and right into the page. Embody what you feel, and express it through the poem.
EX. The tree stands tall and proud/In its little yard/As the days pass/The leaves twist and transform/From green to magnificent gold/Orange/Red/They change identities/And finally/As winter comes/ They silently fall to the ground/Letting go of the branch that held them tenderly/And shrivel into nothing
