There are marks on the inside of the bumper where to drill and mount and as SI has said, use a step cutter, hey make a much cleaner hole. The only time I use a holesaw for fitting sensors is if they are the king with a surround that fits from the outside. If you are going to fit the newer OPS sensors, get the holders that fit inside the bumper. This is the neatest solution as you don't have to have a surround around the sensors. This is the same as the MK6 from the factory. Unfortunately, I had to get the Skoda holders as my holes were already there from factory. I wish it hadn't come with any sensors tbh.