For me it's always the last thing I'll do after working out. I usually take the medicine ball and do 100 sit ups without stopping, then I'll go to a free weight machine, tie a strap around my ankles, add 10 lbs lie down flat on the ground and do a set of fifteen(but I lift my legs instead of my upper body.
Afterwards I'll grab the "sit up bench"(I just call it that because I don't know the real name, it's where you can lie down on incline and wrap your legs around a pad) take a 45lbs weight and do as many sit-ups as I can(do three sets, one minute-two rest in between.)
Hopefully that wasn't very confusing, it's hard to write it down, then showing lol.