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. You can use the assistance of your opposite arm to. 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.
COMPENSATION: Avoid keeping your back rounded or letting your elbow pointing towards the back wall. Keep your shoulder blades pulled back for the entirety of this stretch.