Lay out the areas your going to be drilling with blue Painters Tape (Doesn't leave a sticky residue like normal masking tape when your done)
The tape does several things.. Most importantly it gives you a place to mark your X's for where your going to be drilling. It's been argued by many that while your drilling it also helps to prevent chipping of the gelcoat in the surrounding area. Not sure if I believe that, but what the heck right? Couldn't hurt.
Run the drill in reverse to get the hole started. You it is grinding the gel away as opposed to cutting it. After you've gone in a little bit (The point of the drill is past the gel) flip to forward and punch out the rest of the hole.
If they sell backing plates DEFINATELY buy them, or make some if you have any fabrication skills. Make sure they are flat to the hull.
Use Nylock nuts ONLY when attaching something like this. If one comes loose your putting tremendous loads on small sections of fiberglass which = bad news to say the least. When your tightening them, don't be shy about it, but don't break out a cheater bar or anything as well. Tighten them up tight, but not to the point where your heading the fiberglass starting to creak and crack. If you hear a little that's fine, but stop there.
As other's have said, measure twice cut once.