Hi, Beth! For the Long Step Abs (thanks for the new shorthand!), I have my butt and the tops of the hams on the bench, as well as of course the back on up. I also like to lengthen the legs out away from the end of the step so that there's only a very moderate (but discernible) bend in the knees.
One thing I've noticed is that I have to pay attention to what my feet are doing; at times I try to keep them flat on the floor (which entails pointing the toes) and at other times I try to keep the feet flexed with only the heels on the floor. In that second position, I do have to pay attention to NOT digging the heels into the floor (which takes a bit of workload off the core muscles, believe it or not, when working to keep the lower spine flat on the bench).
I trust I've overanswered your question.
a-Jock