The body forms a straight line from shoulders to feet.
Keep the arms straight the whole time and resist the temptation to bend at the elbows: this exercise helps dissociating scapular movement from elbow movement.
Slowly pull your shoulders backward (retraction), downward (depression), forward (protraction) and upward (elevation) in a controlled circular motion.
Go slowly and take the time to go to the full range of motion in each quadrant.