Once you are able to perform the target reps of the flat knee raises exercise you should move on to the next harder version of the ab working exercise series - The Flat Bent Leg Raises. This exercise is very similar to the flat knee raises, it's just harder due to leverage.
Let's come to the how-to-perform explanation:
Lie down flat on the floor or a mat. Keep your arms flat beside your torso and your legs together. Bend your knees approx. 45 degrees (at flat knee raises it was 90 degrees) and lift your feet slightly from the floor (around 1-2 inches). Make sure that your torso stays stable on the floor during the whole exercise (no hollow-back!) and that your abs are contracted all the time. Slowly bring your legs up over your hips until your feet are above your pelvis (remember to remain your knees locked in the 45 degree position). This move should take 2 seconds. Hold the finish position for a count of one. Now move back slowly to the start position (2 seconds) and start the next rep. If it's possible don't allow your feet to touch the ground during the exercise (if this exercise is to hard, you can slightly let them touch the floor every 3-5 reps)! If you can't manage to meet the target instantly, do as much reps as possible until you can do the requested amount. The 2-1-2 cadence is very reasonable for this exercise (ab muscles have mostly slow-twitch fibres). No shortcuts allowed here (same like the other ab exercises)!
Target: 3 sets of 30 reps