There is only one rule, keep your hand moving.

Don’t worry about grammar, spelling, or punctuation. If you feel stuck, keep writing anyway. Try going back to the prompt and write it out until other words arise, then write those—or, repeat the last word you wrote again and again until new words appear—or, write, “I am stuck,” or “I can’t think of anything,” as long as you keep your hand moving. Writing is physical, the words will move through your body and come out of your hand—just keep it moving.