You simply hold the item up to the body with small clamps and mark your holes. Use a small and sharp center punch to lightly make the dent and drill with a small drill, then up to the size needed. We must have drilled 50-60 holes in the TMF Jeep and all were great. Make sure to use some lube on the bolts that are going into the rivenut tool and have two people when doing the rivnuts as they are easier to keep flat against the body that way. Cake.
If I had to do it again I'd use a step bit, the body material is thin and soft. I made my own rivnut puller with grade 8 bolts and 1/8" thick washers, I also used an impact to pull them in. Did 38 holes and wore out 3 of them. Only spun one but the hole was a little butchered from that big bit grabbing.
We used a 1/2" and it worked perfectly. Some needed a little wobble of the drill to fit, but the tighter the better. We used a wrench and a socket and ratchet with the rivnut tool and we did not have forearm issues, but we wrench daily.
An impact would be awesome, but you would not be able to feel the point where it grabs. It could be easy to go too far. We used red loctite around the rivnut and the body area. not sure it will help, but it cannot hurt.
I would suggest NOT to use a step bit as you can be drilling into other items behind the body and not know it. Just use a normal 1/2" drill and use a rat tail file, or the drill to slightly enlarge the hole. Remeber, you want a tight fit., Not too tight that you dent the metal tapping it in, but stull snug.