Fix a reistance band somewhere. It should have about the height of your hips. Grab it with both hands. Straigten one leg and flex your feet. Lower the other leg down as far as you can go without loosing tension in your abs and without loosing control. Lift up again. The band should assist your leg to manage the weight.
Exercises
Butterfly with Thera Band
Bring your resistance band under tension behind your shoulders. Arms are stretched to the sides. Bring your straight arms together in front of your chest.