Stand for approx. shoulder wide stance (how you prefer).
Keep your knees in line with your toes
Heels should remain in contact with the floor

1. Begin by squatting down as you do like a box squat. Lower your pelvis until you can stay balanced.
At the lowest point find your position with small movements.
2. During standing up, hold your ankle fixed, don't let your shin move back. Drive your pelvis over your toes. (With added weight you must stay over your feet.)

If you don't have enough mobility, use can use a book to lift your heel.