In Brief: Standing in a doorway, place arm closest to wall behind you flush against wall level with shoulder. Rotate torso away from door opening. Hold for 30-45 seconds, repeat other arm.
In depth: Standing in a doorway with back close to door jamb, raise arm closest to wall behind you, and place flush against the wall at shoulder height with thumbs pointing up, locking elbow. Rotate torso so opposite shoulder rotates towards the wall. Hold for 30-45 seconds, repeat with opposite arm, either turning back to opposite side of doorway, or placing arm on interior of same wall.