Bell in ASL looks like you are striking a bell with a mallet and then the mallet is recoiling back after the strike. You take your non-dominant hand open and stand your whole forearm up as if to make the wall of a bell. Then you take your dominant hand in a first, with thumb and index finger both pointing down. The dominant hand strikes the non-dominant hand, then goes back vibrating.
A bell makes a good introductory musical instrument. Babies love shaking a bell and learning how their action can create a sound. Highly motivational objects, like bell, make great early signs because your baby's fascination with the object makes them want to learn the sign.
We can teach bell during the holiday season or as part of introducing our babies to musical instruments, if they attend kinder music or a similar program.