HOW: Start in a seated position with your hands interlocked behind your head. With your shoulders and arms relaxed, squeeze your shoulder blades back. Pretend there is a pencil in between them that you are trying to squeeze. Return to the starting relaxed position and repeat for the prescribed amount of reps.
FEEL: You should feel your shoulder blade muscles working the most in the middle of your upper back.
COMPENSATION: Keep your shoulder and arm muscles relaxed as you squeeze your shoulder blades back.