To sign quiet, bring your index finger to your lips (that is, the universal shhh sign).
Note: This is an abbreviated version of the full ASL sign for quiet. The full version is a two-part sign, where you cross your flat, open hands and then release them for the second part of the sign.
Quiet is a really useful sign for older babies. It is great to be able to get even older kids to pipe down when you are perhaps in a library without needing to yell across the room. Project calm when signing quiet.
You may end up using this sign for a very long time. Use it at church or other social settings like restaurants. This is also a great sign when another baby is taking a nap.