Have something you can lower your body to- a chair, steps, a box, whatever is just about knee height.  Stand up straight.  Lift your right leg to flexion out in front of you.  Slowly lower your body, keeping yourself supported by your left leg.  Be careful to not let your knee go over your toes, so try to point knee out at an angle, and lower your body as slowly and deliberately as possible.  Don’t just plop onto the seat.  Once seated,  you may rise up on both legs and repeat process on opposite side.

When you are ready to advance, repeat same exercise, except stay on same single leg to get back up to standing.