Fall is a natural sign. You start with your open, non-dominant hand forming the ground, palm facing up. Then you form a little person standing upright with the index and middle fingers of your dominant hand in an inverted letter V position on the flat hand. Next, flip the person over, as if they tripped, or backward, as if they fell upside down.
As toddlers are often tumbling over while they are learning to walk and balance, this sign helps keep their tumbles light hearted. "Oops, you fell down. It's okay. Let's try again."
Teach your baby or toddler this sign in conjunction with the actual experience, or when they see this happening to their storybook or cartoon character.