Hi Scarlet,
I have found that when you are Picking your Ab exercises, you want one's that give you Flexion (stretch), and Contraction.
So Crunches with your Feet touching and pulled in (Frog Crunches), and a Rolled up Towel under the small of your Back.
This will allow you to Stretch the Abs before you Contract them, this is a Good Starter, 3 Sets with 15 seconds rest.
Then move to a Bench, and Sit at the end, lay back, and grab the Bench behind your Head or to the Side of your Head.
Now to start, hang your Legs straight off the Bench, keeping them Straight, lower slowly below the Height of the Bench.
Then raise them Slowly until your Hips come off the Bench (straight leg reverse crunch) 3 sets with 15 seconds rest.
Now for the Last, either Hanging Leg or Knee Raises. If you can't do the Hanging Version. You can do the Version off of a Combo Lat tower/dip station, you'll know it when you see it, it has a Back-pad, Armrests, and Handles. Or if you have Access to Ab Straps.