This is the beginner form of the V-Up. If you feel it starts to get easy, then do the more-advanced straight leg version!
Lie supine on your back. Arms are straight in front of of you. Keeping your torso straight and neck long, come up to a V-shape with your legs bent at a 90 degree angle. Pause at the top and contract those abs! And now control yourself back to the floor in supine. Repeat.