HOW: On the side you want to work on reaching behind your back, hold a towel with your palm facing backwards. Hold the towel with your other hand and depending on your range of motion, you can have the towel positioned behind the waist or draped over the opposite shoulder. Using the opposite arm, pull on the towel so that it moves the target arm further behind your back, move as far as you feel comfortable with and hold for the prescribed time and repeat.
FEEL: This should feel like a shoulder stretch, likely in the front and all around your shoulder.
COMPENSATION: Try to limit motion to your shoulder and shoulder blade, try not to lean or bend too much with your trunk and upper body.