Decide on a topic for your poem: a person, animal, object, abstract concept, football team or whatever else you like. With descriptive poetry, it's best to focus on just a single subject. It can be a large entity like a city or a nation, but if you have more than one subject, it dilutes the power of the poem.
Write down a list of words or phrases that you feel evoke your subject. Physical descriptions are good, but you can also focus on smells, atmosphere, personality or the emotions it invokes in you. The words you choose will form the basis of your poem.
Determine a structure for your poem. You can used a well-established structure, such as a sonnet or a limerick, or establish a structure of your own. Decide on the meter, rhyme scheme (if any) and similar structural elements. Don't be afraid to tweak it as you go. There's nothing that says you can't change your structure in the middle if you find a better way to make it work.
Write a draft of your poem, using your established structure as a guideline. Apply the words and phrases you gathered in Step 2, arranging them in an order that pleases you and that contributes to the poem's elegance. Like a piece of prose, a poem should usually build toward its finale, so save your best descriptive phrase for the end. Don't worry about making everything perfect the first time. The act of getting it all on paper is more important at this stage.
Revise your poem and make editorial changes as appropriate. Smooth out awkward passages, rework the rhyme scheme until it is to your liking and polish the structure so that it embodies your intentions as well as possible. It may be a good idea to let the poem sit for a few weeks before beginning your edits. The time grants you some perspective and objectivity. If you have any friends or colleagues with a knowledge of poetry, you may also wish to give it to them and ask for constructive comments.
Spell-check your poem and make any last-minute corrections. Some poems may involve deliberate misspellings or grammatical errors--if correctly applied they can be quite evocative--but make sure that any such "errors" are present by choice.
Send your poem to your friends, or mail it to a poetry organization to see if it would be interested in publishing it.