Final answer:
To learn how to sign "78" in ASL, make the handshape for 7 and bounce your hand down toward the floor as you make the shape for 8.
Step-by-step explanation:
The correct instructions to learn how to sign "78" in ASL is to make the handshape for 7, then bounce your hand down toward the floor as you make the shape for 8. This method helps to differentiate between the signs for 7 and 8 by using different hand movements. By following these instructions, you will be able to practice signing "78" accurately and prepare to surprise your grandma on her birthday.