Sounds like a great project dude. First, if you are not already doing this already, I suggest that you take the rear bumper off before you do anything. There are a lot of plastic pieces and braces back there that you may have to strategically avoid. As for routing the wires, there should already be a location to bring the wires through in the chassis, maybe the shop manual has more info on that.
Before you drill the hole, cover the area with tape and mark it. The tape will help stop the plastic from possibly chipping or deforming, I suggest tuck tape or masking tape. I would hope that the drill that they supplied you is a hair smaller that what you will need as it is easier to go at the hole with a round edge file or dowel w/sandpaper to make it bigger than it is to make the hole smaller

. Also be sure that when you do have the placement correct, drill a pilot hole first.
Hope those suggestions help. Good luck! I have a question though, are all 4 sensors needed, or could 2 be utilized, say on each side of the license plate? Then maybe mount the other two up front to give you audible front bumper warning. That would be handy as I am am very paranoid about bumping my lower bumpers on curbs! I always get out and realize I am like 1 foot from the curb lol.