They're quite simple really: using a decline bench you lower yourself til your torso is parallel with the floor, then hold that position for a 5 count, then perform a situp. Use weight if you can - like any exercise the goal is always to add weight when possible. I've seen NS advise 5x5 as the set/rep scheme, and I imagine other schemes would be effective. Personally I think 5's are great for developing a solid core for deads/squats etc.
there really isnt a set rep scheme, I suggest with starting with 5x5, but personally I worked up to 5x8 reps with a 45lb plate, and its done wonders for my abs, I've never been on stage with anyone who had ones that could compare