Bib in ASL looks like you are outlining a bib. Take both hands and make them into a fist with your index finger extended. Start outlining the big with your fingers together down at belly level, move them apart, up your body to neck level then bring the fingers back together.
Bib is a great sign to add to your eating routine. When you are getting your baby ready to eat, put their bib and make the sign. Over time, your baby will learn the sign. Some babies will even ask for the bib because they think it means they will get food.
Bib is a great sign to teach your baby, not only to signal eating time, but eventually for your baby to associate it with staying clean while eating.