Soap is signed by taking your non-dominant hand and holding it flat in front of you, with palm up. Then, place your dominant hand a few inches above and flick your fingers down so they touch the palm of your non-dominant hand. The motion looks like you are building up a lather.
Soap is a good intermediate sign that can be taught for use in the bath, or when toddlers are learning to wash their hands. Learning a sign like soap can help make these daily routines easier, because your child can work with you in getting the task done (lathering up with soap), instead of feeling helpless.
Bathing is a favorite baby activity. We use soap contextually by buying our babies fun soap shapes.