To make the baby sign for sing, take your slightly curved open hand, hold it close to the top of your chest, then move out outward and upward at an angle, as if indicating the air that is carrying the sound waves generated by your singing.
Alternatively, you can make the sign for music, so take your non-dominant hand and hold it out across your body. Then take your dominant hand and wave it back and forth across your non-dominant forearm, as if you are conducting a choir.
Use the sing sign before you do some singing with your baby. Singing some songs is a favorite activity with babies, even if they can't sing the exact words. Hearing the same songs over and over again will help your baby learn to speak - as well as drive her parents nuts!
We use the sign for sing when our babies begin to develop verbal skills and musical sensibilities.