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, starting 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.
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.
The official ASL sign for hello is like a little salute. You can also use a little wave, like the more traditional gesture for saying hi.