You’re going to need a yoga block and a light dumbbell (in and around five pounds is perfect).
Step 1: Assume the quadruped position (meaning get on all fours).
Step 2: Place your left hand on a yoga block and grab the dumbbell with your right hand.
Step 3: Scoop your right shoulder blade into position by protracting your shoulder blade and make sure your spine is in a good position (aka its nice natural curve is present in all its glory). In this position, that dumbbell should be dangling off the ground a little bit.
Step 4: Keeping that right arm straight, drive your hand back behind you, putting your shoulder into extension. Keep going as far as you can, using your breath to help you. As you do, make sure that shoulder blade is kept in position the whole time. Relax and repeat.