Face a wall or trellis and climb up it until you are in a handstand position facing the wall
Squeeze your abs, hamstrings, and glutes together to get your hips to rotate backwards, eliminating lumbar curvature
When you return to the starting position, exaggerate the opposite movement, maximizing your lumbar curvature so that you can feel the difference between one position and another.
Handstand with pelvic retroversion - beginners
Handstand with pelvic retroversion - intermediate