To sign doll, curve your index finger into a C-shape. Then rub it down your nose twice.
I remember this by thinking about rubbing a doll's cute button nose. It is so cute, you need to rub the nose twice, and curve (or make the letter 'C') with your index finger.
If your baby has a favorite doll that she loves to play with, this is a good sign. Do the doll sign whenever you give her the doll or whenever you see her playing with the doll. "Are you having fun playing with your doll?"
Both girls and boys love to play with dolls and imitate adult interactions. Teach the sign for doll as soon as your baby begins to show preferences for certain toys.