When I was in junior college, Dick had me do lots of hyperextensions. You really never see people do them anymore but my spinal erectors looked like my butt went up my back.
Hypers are a real gem, but we found that the upper half was helpful for building the back and the lower half (if this makes any sense) was not so important. So, we focused on a full high position and control to about halfway down (actually more than half) then squeezing back up. The bottom part was all momentum and seemed to over stretch me sometimes. You can ignore this advice, if it doesn’t make sense, but the concept helped me.
As for sets and reps, I just do/did them until I feel my spinal erectors fill with blood. It is like a balloon filling up; each rep, they just seem to load up. 5-8 reps at most, I see no need for more.
One thing you need, however, is someone you can trust. I have done them by myself by doing an upside down lat pull, but it is much better to have someone raise the bar or plate to your head.
Having a lower hyper bench is the best way to help your spotter. At the PBBC we used a hyper set-up that was about two feet off the ground. You can use the O bar with weights, but you will find that you need to stick to 5’s, 10’s and 25’s with a lower hyper, otherwise the weights hit the floor. Higher than a few feet, wouldn’t be a problem, I guess.
We also had 102.5 pound “blue plates.” These were great for workouts. Dick would lift them up and I would hold the top position than do reps.
I see girls at spas doing a hyper like exercise, but they go fast and don’t add weight. It would kill your lower back after a few weeks. I think weights make you stay under control and do a better hyper. I would recommend using weight probably from the first day, just to do the lift correct.
So, get a buddy to load up the weights across your neck, just like squats, and start hypering.