Sit with your back against the pad and extend your legs fully, then lower them slowly to keep tension on your quads.
Feet lower on the platform emphasize quads, higher targets glutes and hamstrings. Narrow stance hits outer quads, wide stance hits inner quads.
Keep your hips down and curl your legs toward your glutes, then lower them slowly to maintain tension on your hamstrings.
Toes pointed inward emphasizes outer calves, toes outward emphasizes inner calves.