Find your meter. Blank verse depends on using a certain meter (iambic, trochaic, spondaic) If you haven't learned your meters, look them up. Lambic is an easy one, it consists of two syllables with the second one stressed, or in layman's terms, da-DUM, da-DUM, da-DUM.
Find your length. Blank verse is classically written in pentameter, so that there would be five instances of your meter in each line.
Find your theme. It's pretty difficult to write without a general poetic thesis and if you do it will probably come out sounding like gibberish.
Follow your muse. This critical part of poetry writing is the hardest thing to advise. Either the inspiration comes to you, or it doesn't. Think about your theme and words that would describe your feelings. Then try to use them in your lines.
Keep the iambic pentameter just the slightest bit off-kilter. Classical examples of blank verse sometimes bend (or break) iambic for a couple of beats, particularly when using the names of places or people, sometimes adding extra syllables.