I drilled a pilot hole in each of the little squares to start and then drilled a larger hole with the largest drill bit I had (about 3/8 inch). Then I used the semi-tapered Dremmel grinding bit which allowed me to get pretty well into the square corners--they are not perfectly square, but very close. You may have to reach behind the grille after grinding and scrape off some of the material when you are done as some will adhere to the edges of each hole after grinding. I just used my hand and rubbed it all off when I put the euro duct work in behind the grille. As I mentioned, it's very stealthy and has worked wonders for brake cooling at the track.