To sign flat, bend both open hands and hold them one in front of the other. While keeping your non-dominant bent hand steady, move your dominant bent hand forward and away from the other hand, indicating the length of a flat surface, like a street.
Teach your toddler the flat sign in the context of describing a flat surface, like a road, or a piece of furniture.
Teach your toddler this sign in conjunction with flat objects that are easily accessible or observable to your curious child. The sign is an abstract concept, so the more applicable objects you can show that are flat, the sooner your toddler can grasp the idea.