Right the first step is to be able to do a basic sit up using your stomach muscles and not your butt (yes seriously) 
I always find that there is no way that i am ever going to manage a situp to start off with so start with a situp frame thing to help. Knees bent and wiggle your toes as you sit up. Make sure you pull your stomach in as you are sitting up too. It all means you are using the correct muscles.
Also do these with your knees rolled to the side equal numbers for each side.
One you can easily do this with the frame do it with out the frame.
Next stage is to lift feet off the floor and hold shin and feet parallel to the floot. if you struggle to start off with use a sofa to rest them on.
Once you can do these then the evil evil exercises start. you need a ball and a person to throw catch with or a door to bounce off.
Phase one Sit in up position of situp throw ball and catch. Lower as fraction towards floor and hold, throw ball and catch. Lower a fraction more to the floor, hold whilst throwing ball and catching. Repeat lowering to floor, holding and catching until lying on the floor. Breath and take pain killers.
When you can do all the way done once confortably repeat so you do it twice. Build it up til you can do this 5 times down.
There do it in reverse. Start lying on the floor, throw and catch. lift fractionally off the floor, hold whilst throwing and catching. Keep repeating until sat upright.
build up until you can do five of these.
THEN do from a round repeat of the throw and hold exercises starting from lying down, until you get into the upright position and then back down to lying down again.
This is not a quick fix and needs slowly working into. but if you take it gently it helps. Also take rest days between doing the throw and hold exercises. you will feel them. It burns and hurts alot. But then again pre surgery I had a stomach that went inwards from doing them. Post ds I have only managed a stomach that can control its own shape better but havent down the exercises properally or consistantly.
DO NOT do this if you have been advised not to do situps for medical reasons.