New OEM bumpers should have "(X)" markings on the back showing where to drill the holes. You just have to be sure you drill the US market bracket holes, not the Japanese ones.
For an already mounted bumper, I say skip drilling holes and find another way like discussed
HERE.
If you REALLY want to drill holes and don't want to pull the bumper I say use a tape measure to find the center of the bumper. Painters tape will allow you to easily mark the spot with a sharpie. Then you can measure the distance of the holes needed, center to center, mark them on the bumper using the center line as a halfway point, and drill. Cover everything with painters tape to prevent a slipped drill bit or another unforeseen occurrence from scratching your bumper.
I have never installed a from plate bracket (Cali cars always come with them dealer installed) but if I was forced to this is how I would do it.