Writing a descriptive poem requires that you have a subject to write about. Once you have that subject, start the poem. Keep the words simple. Don't over-complicate the poem by trying to make it sound like a poem. The best poetry is easy to understand. You're not crafting a riddle or a puzzle. Don't make your reader search for meaning.
Now that you have your subject, decide what you want to describe. Is the setting the important element? Is it the way the rain strikes the narrator's head like little pebbles? Is it the short skirt on the woman sitting across from you in the subway? Pick the element that needs describing and work it into the poem, describing every aspect necessary. Use multiple sentences to describe the way that skirt slides up her thighs as she looks over her shoulder or the way the rain drops run down the narrator's back like ants.
Now that you have your subject and the descriptive element, close the poem. Stick with one descriptive element and one subject to practice this style. Bringing in too many descriptive elements can cloud the reader's vision. Even if there are 32 people on the subway next to her and each one is important to the story, she is the main element worth describing. Don't waste words on other passengers' clothing or mannerisms.
Edit; proofread; cut. Go back and cut out any words and sentences that are not absolutely necessary. Be judicious. Though you are describing the icy rain in your poem, if it's not the subject of the poem, don't let the description overpower the subject. Your poem can be a reflection on a love affair and the narrator just happens to be standing in the rain. Next time a reader stands in the rain, if you describe it truthfully they may very well think about your poem because it was so clear, so real.
Set the poem aside for a few weeks. Then return to it with a fresh perspective. You are your best editor. As you write a poem, you are more attached to the words because you just thought of them. And in your head, everything you wrote makes sense. But when you return and read it again in a new frame of mind (which is how all readers read your poems) you will see the holes or inconsistencies that previously made sense.