Lay on your back, with your hands anchored behind your head. Start with your legs bent and knees directly above the hips.
Engage your lats & core by actively pulling with your arms, pressing the lower back into the floor & driving the elbows forwards.
Now maintain tension as you extend your legs out. Adjusting the angle at which you extend your legs will adjust the difficulty. The lower your legs are, the more challenging it will be, but make sure that your lower back remains in contact with the floor. Pause with your leg extended, then bend the knee back to the start and repeat for repetitions.