1. "C" handshape: Make a "C" shape with your dominant hand, with your thumb pointing up and your other fingers closed.
2. "Animal" sign: Bring your "C" handshape to your chin, palm facing inward, and then move it downward as if you are "milking" a cow.
This is the most common sign for "cow" in ASL.
Note: There can be regional variations in ASL signs.