Lie down in a supine position with arms straight at your sides. Bend your right knee until your right foot is flat on the floor. Hold the left leg straight during the whole exercise.
Lift the pelvis and the straight leg at the same time until your core forms a straight line with your left leg.
Return to the starting position and repeat. When done, switch legs.
If you want to make it more challenging, keep your butt from touching the floor.