Lie on your back and bend your knees at 90 degrees.
Pull your toes in towards you and rest your weight on your heels.
Now tuck in your navel and lift your pelvis off the floor.
Tighten your buttocks and your abdominal muscles and keep breathing normally.
Make sure your back is in line with your thighs, so you can draw an imaginary line from your shoulders through your back, pelvis and knees.
Make sure you keep your pelvis level during this exercise, hold for the required time and then lower back down in a controlled manner.