Lie on your back with your knees bent and your feet flat on the floor.
Lie with your arms by your side.
Tighten your abdominal muscles to flatten the small of your back against the floor.
Tighten your buttock muscles and tuck your tail bone underneath you.
Lift your hips off the floor until you have a straight line from your shoulders to your knees.
Make sure you keep your hips level throughout.
If this is difficult, lower your hips back down to a level where this is easier.