The two center mounting holes in the block fit a standard reflector bracket. The two off to the side fit my Topeak rack's bracket, YMMV.Use a drill bit to clear the support layer over the nut wells, then press #6 nuts into the place left for them (it's tight intentionally - I used a pair of pliers to press them in). I had to use 2" long screws and cut them down so they didn't stick out the back and hit the reflector bracket.Put the two halves together around the pipe and secure. On my rack, two of the screws hit if screwed in too far and two did not, so I snugged up the two that hit as much as possible then tightened with the other two.Use washers and the sheet metal screws to secure the block to the rack's reflector bracket.  I had to remove the screws from the stays and swing the rack up away from the fenders to get access to the back for this step.FYI that GoPro mount is excellent (better than what came with my camera, and lower profile since it has to allow clearance for the bag to go on and off.  You can find it here: http://www.thingiverse.com/thing:320681