HOW: Begin in a seated position. Place your arms behind your back. Keep your arms relaxed as you squeeze your shoulder blades down and back. Imagine trying to squeeze a pencil in between them. Repeat for the prescribed amount of reps.
FEEL: You should feel your shoulder blade muscles working as you squeeze them back.
COMPENSATION: Keep your upper traps and the arms behind your back relaxed. Focus all of your energy on squeezing your shoulder blades.