I did a few of these. The method that worked best for me was getting 3/8" threaded rod and cutting it into 8 - 8" pieces. Found my reference points for the bracket and drilled holes for the threaded rod sections. I injected masons glue/epoxy into the holes and put a bit on the rod sections that I inserted into the stones. You'll learn to keep the gluing to a min. Too much clogs your threads. Took my lunch break. The glue was like a bondo mix. Took 20 min to set. Now, with my anchors in place I would have to 'level' the surface. If I was on an uneven surface, I would stack 3/8" fender washers on the anchors that needed them to level the surface. The bracket would be put on the anchors, and be bolted down. Keep in mind that this looks best when pre-wired so the wires are concealed. If it's an existing/retro job, The pictures above explain what I would do too. Getting a cabinet guy in to help out rarely fails.