Pinch your fingers together on each hand to form a bent 'O' hand in ASL, then touch your two hands together, twisting your hands between touches.
Once your child starts counting, you can use the sign for number to prompt them to count something. To teach the sign, you sign number before counting something. "What's the number of balls?"
After teaching the sign for number based on counting in real-life situations, introduce the flash card to start to get your child associating the symbols for numbers with the concept.
Get all four volumes of Baby Signing Time...