Bye-bye is the same as the traditional gesture for the word. Open your palm, folding down your fingers, then open your palm again. An ASL alternative for goodbye or bye-bye is to wave your open hand sideways, like a leaf swaying in the wind.
Bye-bye is a good, initial sign that you can use with your baby and friends. As guests or family member leave the house, they sign goodbye to your baby, and your baby learns to make the sign back.
This is a natural sign. Your baby will learn the motion through imitation.