There are a million different ways to write, and the best part about writing is you can't really do it "wrong." Encourage your child to write a story or a letter to a friend, journal their feelings, or document everything they did in a day to get them going. You could give them a prompt for each day to help them keep up with the hobby, or encourage them to write a story in chunks and then perform a reading of it to the whole family when it's finished.