HOW: Place your hand behind your back as high up as you feel comfortable. Ideally you will be able to reach the opposite shoulder blade. Once you find this point, stand up tall while pulling your shoulder blades together. This will help stretch the shoulder into this internally rotated position. Slowly move back and forth from a rounded to an upright position. As you feel more comfortable you will be able to elevate that arm behind your back higher and higher.
FEEL: You should feel the front of the shoulder being stretched with this exercise.
COMPENSATIONS: Avoid keeping your back rounded or letting your elbow point towards the back wall.