Best way is to remove rear plastics, bolt in the brackets (Loosely) and find the cutting hole measurements, cut your holes, put rear plastics back in, with the the brackets protruding through the holes, with the brackets still loose, bolt on the middle bar, and tighten the brackets up going through the speaker holes, I bolted both brackets up tight and the brace didn't fit, so had to loosen them for some movement, it's designed to have a secure fit. Or you just try locate a place for the hole going through the speaker hole, but it's very tricky, removing rear quarter plastics even though not completely is the easiest i found, fitted my spoon brace about 2 weeks ago.
I would recommend waxing your inner arches since you'll be in there, rear arches rust from the inside first!