This is the core section my trainer gave me:
Leg Raise – Lie on your side on the floor. Raise both legs off the ground while keeping
them together. Lower your legs slowly back to the floor. Perform 1 set of 10, then switch
sides. You can also perform the leg raise while lying flat on your back.
Leg Raise with Knee Bend – Lie flat on your back and raise your legs a few inches off the
ground and bend your knees, slowly bringing them to your chest. Reverse the movements
by unbending your knees and slowly lowering your legs to the ground. Perform as many
sets of 10 as you are comfortable with. If these are performed correctly, you will feel it in
your abdominal region.
Leg Roll – Lie on your back with your legs stretched out to either side. Pull your knees to
your chest. Roll your legs to the right while keeping your shoulders flat on the ground.
Perform one set of 10 and repeat with the left side.