To sign play, for the ASL letter 'Y' with both hands: stick your thumbs and pinkie fingers out, while curving the rest of your fingers in. Then wiggle both 'Y' hands around and in front of you, as if suggesting a bunch of kids playing or running about.
Teach this sign so that your baby or toddler can know the general concept of play in its many forms. You can also teach it while you are dressing your baby up, getting him ready for a play date with friends.
Teach play in the context of a real-life scenario, not just using the flash card. It is easiest for your baby to connect the concept to actual play dates, or when you play with him or her using their toys and other games.