Lay on your back, with hips and knees at 90 degrees. Lift your shoulders off the floor as you bring your ribcage into your hips to shorten your core (protraction and depression of the scapula).
Reach with your hands forwards as far as possible while pushing the palms into the floor. This position should make your lower back touch the floor - make sure your lower back stays in contact with the floor at all times!
Once set up, straighten your legs out in front of you. The lower you extend them the harder the exercise get. Aim to go as low as your lower back allows you. Keep your core short and engaged at all times. If the exercise is too difficult to perform correctly, move you legs at a higher angle.
Do not let your feet drop when bringing your knees in. The feet need to move in a straight line.