Lie flat on your back. Extend your arms so they're against the sides of your body with your palms pressing into the floor, or bend your elbows and place your palms under the back of your head. Bend your knees and draw them into your ribs. This will make it easier to pull your navel in toward your spine and actively press your lower back flat on the ground.
Lift both legs straight up toward the ceiling, continuing to engage your abs and pressing your lower back into the ground. Keeping your core strong, slowly lower your right leg down toward the ground until it is a few inches above. Then slowly scissor your legs, lifting your right leg back up as you lower your left leg down toward the ground.
You may not feel this until you do it for a while, so repeat this move for a total of 20 to 30 times on each leg (or more if you're still not feeling it).