Wherever space and time interact, there is information, and wherever information can be ordered into knowledge, and knowledge can be applied, there is intelligence.
Pavel Mirsky, mid 21st Century Russian General
I did not have to drill mine - but I will mention that I have been quite impressed with the function and durability of the step-drills offered by Harbor Frieght:
First off a question. How are you planning on attaching the mirror to the door skin? I ask because the mounting I bought from one of the usuals was rubbish. It consisted of a plastic pad with two pockets the fit into the two screw holes. In theory running the screws into those pockets would expand them, holding the mirror firmly in place. In reality, the mirror flopped around and fell off when I shut the door. I wound up JB Welding two nuts to the underside of the door skin. This can be accomplished from the outside of the door, unless you have really fat fingers.
That’s what I’d use for the big hole. I don’t know the sizes just now but as luck would have it I was planning to remove the mirror this afternoon to see if I could increase the spring tension on the arm. I’ll measure then and get back to you.
The smaller the hole the easier it is to drill precisely. Once you exceed a quarter inch or so you need to exercise caution and go slow and steady. If using a stepped bit at higher speed and just a bit too much pressure you risk the sharp edge cutting too quickly and binding the bit, puckering the sheet metal in the process.
Draw the pattern on the outside of the door with a fine permanent marker then go slow as she goes. I prefer to finish making larger holes in sheet metal slowly with a carbide dremmel bit.
If drilling over painted areas that you want to preserve, consider covering a large area with blue masking tape. And a double layer right at the hole locations.
This double layer of tape at the hole locations will give something for the bit to grab initially and help keep the bit from drifting from exactly where the hole should be. The larger area with the single layer of tape will protect the paint in case the bit does drift.
Once you have the location for the holes on one door, I’d suggest to make a paper template that outlines the lines of the door and holes. That makes it easy to transfer the holes to the other door by simply flipping the paper over.
I would have your friend sit in the vehicle and test locate the mirror position for maximum visibility and adjustment. The passenger side can be particularly tricky based on the limited range of adjustment of some mirrors.