Just in case you are wondering, what I did is measure an old door I had for the car, then I marked where the holes should be. I also measured the mirror hole spacing, then I drilled the front hole (ONLY THIS HOLE AT THIS TIME). I installed the rivnut, screwed the mirror to the door and check to make sure it lined up with the mark I made previously (it was off slightly). I then used a transfer punch (get a whole set at harbor freight cheap). Then I removed the mirror and drilled the second hole and installed the rivnut.
It isn't hard, but it isn't something you want to screw up on since there is not wiggle room really (maybe a 1/64th or something at best). Due to the various belt/body lines it can be deceiving when measuring. You can also make a simple template out of some construction paper or printer paper, but I highly recommend you measure many times and drill once.