Start from a bridge. Your knee should be at 90°.

1. Extend one of your legs without letting down your pelvis
2. Go back to the bridge
3. Extend your other leg
4. Go back to the bridge

Torso and thigh in one line, pelvis raised
Permanent ankle flexion
Support on your scapula (shoulder blades)
Movements just in the knee
The pelvis and thigh are fixed
At the top, squeeze your glutes