Start on your hands and knees with your hands under your shoulders and knees under your hips.
Your knees should be hips width apart.
Keeping your back straight tighten your abdominal muscles.
Lift one leg up behind you, keeping your knee at 90 degrees.
Think about pushing the sole of your elevated foot up towards the ceiling.
Control the movement as you bring your knee back down towards the floor, then repeat.
Ensure you do not arch your back or twist your hips as you do this.