HOW: Grab a weighted ball or object. Bring your arm into the 90/90 position with your elbow at shoulder height. Try to throw the ball straight up by rotating your shoulder towards the sky. Catch the ball and then quickly throw it back up again.
FEEL: You should feel all the muscles in the back of your shoulder working, in particular, your shoulder blade muscles.
COMPENSATION: Sometimes its easier to hold an equivalent weight on the other arm, even if you are not performing the exercise on that side, as it helps with balance.