A potted guide to body retaining nuts and their application in etched brass model locomotive kits:
1) Remember to fit them. First. Before you build the body. If you don't, normally the won't be any access to do them afterwards. This leads to frustration, bad language and trying to take the thing apart and bending the etched parts. Guess how I know this.
2) Make sure the bolt will go through the hole. They normally won't and you have to ream out the hole. It's better to discover this before you solder the nut in place. Clean the burr off the edge of the hole or the nut won't sit properly.
3) If possible, line the hole in the nut and the hole in the footplate up by eye. Tack the nut in place and see if the bolt goes through. If not, heat the join, poke the nut and try again. This takes time but if you can't do this, the alternative is painful.
4) If step 3 doesn't work then you'll need to bolt the nut in place and then solder. Try to use as little solder as possible. Make sure the threads of the bolt are covered in oil or pen to stop the solder sticking. It's no guarantee but it might help. If you do solder the bolt into the nut it's worth heating the joint and trying to unscrew it while the solder is molten. If that doesn't work you'll have to cut the nut and bolt off and start again. Makes you want to spend more time on 3) doesn't it ?
5) Run the bolt back and forth through the nut. Sometimes you'll find the hole alignment isn't spot on but the bolt cuts a thread in the brass. Engineering types will use a tap for this part but if you don't have one, a steel bolt isn't a bad alternative.
6) Solder the but fully in place. When you've got a complete body, having the retaining nuts come loose because you have only tacked them in place is A Bad Thing. Guess how I know this.
7) Do step 5) again in case any solder has found its way into the join. It happens, trust me on this.
8) Clean the join of flux. It's harder to get at it later.