Place a bench, box, or another stable object underneath the pull-up bar. Stand on top of the bench, and grip the pull-up bar with both hands. Your hands should be slightly wider than shoulder-width distance apart, and your palms should be facing away from your body.
With control, remove your feet from the bench, and hang from the bar. Your shoulders should be relaxed.
Without bending your arms or swinging, perform a reverse shrug to depress your shoulder blades. Pause for a second at the bottom, then return your shoulders to the starting position.