Pick a topic. Select a topic that is easy to write a poem about. Pick one that inspires you so that the words will come easily.
Write the poem in English first so that the words flow naturally. Don't worry about rhyming; the words are not likely to rhyme once translated. Once the poem is finished, proofread it and make any necessary edits.
Consult your dictionary to find Spanish translations for the words. If you find a word that has multiple Spanish translations, choose the one that sounds most "musical" for your poem. Use an online translator (like BabelFish or Google Translate) to see what your new Spanish poem says in English. If it no longer makes sense, go through word by word to correct any errors.
Resist the urge to write the entire poem with an online translator. Computer translations are often inaccurate, sometimes providing comical results.