Ferrules normally fit pretty tight. Try a test hole in a piece of wood. Drill it all the way thru, so you can push the ferrule back out.If the guitar already has a finish on it, you may want to grind the holes a bit to keep the ferrule from "pushing" the finish. Use a cone shaped grinding bit,and be careful not to take too much. It won't effect how tightly it fits. If it has a good coat of polyester, or whatever, and you don't cut the paint back a bit, the wood will expand enough to pop the finish up. It'll look like 6 little fisheyes around your ferrules. It''ll ruin your day....
This goes for any hole that you are going to put a screw into. Bridge studs, tailpiece studs, strap buttons, pick-up bezel screws. Yeah, most of these won't be exposed anyway, but the lifted finish can spread. If the finish is very thin, this usually won't happen, but suppliers mostly lay the paint on real heavy to make it look extra glossy. It's like working with an egg shell, brittle and easily cracked.
If you have access to a drill press, put a bolt in it (thread first) and cover the blunt end with tape. You can use it as a press to gently push ferrules and studs into place. It's much smoother and less risky than tapping them in with a hammer. Good luck!
