Set up a glute-ham bench or similar to place the pad under your upper hamstrings. Lean back under control as far as you’re able, and then crunch the abs and sit back up as you rotate your trunk to one side, alternating the direction of rotation each rep.
 
Tight glutes in the bottom will help prevent the hyperextension of the lower back that commonly causes pain in this exercise.
 
You can also limit the range of motion if the full movement bothers your lower back, or as a way to make it somewhat more difficult, by stopping with your upper body approximately horizontal.
 
Notes
This can be an extremely taxing exercise for athletes who have never performed them previously—ease into them with a very low number of reps and sets (e.g. 3 x 5) the first exposure to gauge your response.
 
Programming
GHD rotating sit-ups should be done at the end of a workout and can be done for sets of anywhere from 5-20+/side depending on conditioning.
 
Variations
You can pause in the horizontal position or bottom, use slower tempos, and add weight.