Signing: To greet someone with a hello, make the sign by extending your fingers and cross your thumb in front of your palm. Now take the hand, and start with your hand in front of your ear and extend it outward and away from your body.
Finally, smile! Emotional signs like hello are learned as much from your body language as from the actual signs.
Usage: I find it more natural for a baby to just simply wave hello and goodbye. However, if you want to use the official ASL hello sign, do the sign every time you greet someone, and encourage them to make the sign back. Also use the sign whenever you see your baby after even a short absence, while saying a cheery hello.
Flash Card: Click the link to view the Hello Baby Sign Language Flash Card. The flash cards are printable and available in both U.S. Letter and A4 sizes.