Use a step or box that when you place your foot on it, your knee will bend at a 90-degree angle. Place your entire foot on the box and lean slightly onto the “up” leg to load your body weight on it.
Push through your heel and bring your “down” leg to meet your “up” at the top of the box, standing tall and erect. Return to the starting position by lowering the same “down” leg back to floor under control. Repeat.