HOW: Start by lying on your back with a yoga block or some other firm object in between your feet. Keeping your core engaged, squeeze the block between your feet. Now while maintaining that squeeze on the block, slowly curl your knees towards your hips, then lower.
FEEL: You should feel the muscles in your hip working while you squeeze the block. You may also feel your hamstring muscles behind your thigh working as you bend your knee.
COMPENSATION: Do not arch your back. Keep your ribcage down and your core on.