OK, first, you're gonna have to lose the tow hooks. You need to remove the covers from the bumpers, they have tabs on the back that pop through the metal. On the back, you might as well break them off, as they won't line up later. Then remove the bolts that attach the bumper to the brackets. There's one on each side. Decide how much you want to shorten them. I shortened mine about 1 1/2", IIRC. Mark the locations for you new holes now, before you cut anything off. Take a sawzall/hacksaw/angle grinder and cut off the proper amout (mark it first) off each side. Make sure you cut the same amount off each side. Then it's time to drill ze holes. Bolt the brackets back on to the bumpers and put them back on the car. On the rear, you need to flip the bumper upside down (with the brackets normal) so the bumper isn't too low. This renders the tabs on the cover useless. You need to screw the cover on in discreet locations, or come up with an alternative method .
Here's the after pics of mine: