The Baby Sign Language sign for rattle isn't actually part of ASL -- but don't let this stop you from using it with your baby! Take your dominant hand, hold it in a modified fist with your pointer finger hooked and touching your thumb, then shake it back and forth, the way you would shake a baby's rattle.
Use this sign when you have an actual rattle on hand, to show your baby that he can ask for the rattle by mimicking your sign.
Does your baby have a favorite rattle? We introduced the sign by first shaking the rattle for our baby and then showing our baby the sign. If your baby does not favor a rattle as a regular toy, consider introducing the sign when your toddler joins kinder music programs.