HOW: Start with your entire forearms against the wall. Push into the wall with the pink side of your hands as well as your elbows the entire time. Slowly work your way up the wall, then slide back down. Repeat.
FEEL: You should feel all the muscles working in your shoulder, especially the muscles under your armpit. Those are the muscles we want to work the most.
COMPENSATION: Do not let your back arch. Keep your core engaged the entire time.