They mount to the body.... mainly the pinch weld... they also have arms welded to the step that keep it from leveraging back when loaded by pushing up against the cab bottom.
You'll have to drill the pinch weld in a couple spots..... helps to have another person helping hold the step while you lay it out.
The mounting seemed cheesy to me, but once they were on, they were rock solid. No flex or bending, and I'm a little north of 300lbs
You should have used vise grips. I have the RBP steps which is exactly the same as N Fab steps (N Fab makes it for RBP and RBP slaps their name brand on). It looks the best of all steps even with my lift on. You'd be better off getting the N Fab steps, not the RBP for the money. I got mine for the same price as the N Fab.