Stand with your feet about hip-width apart and knees slightly bent. Place a body bar, light barbell, or stick vertically along your back and hold the bar with one hand at your lower back and the other hand above your head. The bar should remain in contact with your back throughout the entire movement.
Hinge forward at the hip, as if you were trying to get your butt to reach the wall behind you, while still keeping your back straight. Your knees shouldn't bend anymore than the slight bend that's already there. Dig your heels in to the floor and straighten back up.