HOW: Grab a weighted ball or object. Straight your arm out to the side in the shape of a T. Your elbow and hands should be in a straight line at shoulder distance. Try to throw the ball straight up 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.