Start with a blank page and brainstorm your idea. As with any art medium, your imagination is your only limit. Free verse poetry requires nothing but grammatically correct text and a clear message. The clear message is debatable, though. Many poets write opaque, abstract poetry wrought with so many metaphors and symbols that the metaphors carry more weight than the actual idea they represent.
Pen a rough draft of the poem. The writing process is different for every poet, but essentially boils down to two things: writing and editing. As you write, don't worry about line breaks, rhyme scheme, anything. In fact don't try to rhyme. If you find yourself at the end of a line, millimeters away from writing a word that rhymes with the end word on the above line, don't. Grab a thesaurus if you draw a blank and find something else. This isn't necessary, but it furthers the free verse theme. You can rhyme, but you don't have to.
Go back and re-read your poem, several times. Cut out all unnecessary words. They clutter the message. If you wrote a metaphor you just love but it has nothing to do with the poem, save it and write a poem that caters specifically to that metaphor. Check for basic writing mistakes: passive voice, misspellings or mis-used words.
Scan your poem, continuing to ignore rhyme scheme and line breaks. In fact, you don't have to have any line breaks. Look for weak words, or words that sound awkward. Replace them if possible with interesting words or words that sound good together. Though you are not prescribing to an end line rhyme scheme, sound is important in any poem; sound distinguishes a free verse poem from short prose. Don't stress over end-rhymes, but consider assonance and alliteration. These are much more pleasing to read and listen to than end-rhymes.
Leave the poem alone for a few weeks or longer. Come back to it and edit again. Using this fresh perspective allows you to read your poem more subjectively as a reader or editor may.