What you need:
Pattern of some kind
What you do:
1). Print a pattern and cut it to size/shape
2). Tape pattern in place
3). Using x-acto blade, trace the pattern on the paper (cut through if you can, to make sure you get an outline on the back surface of the mirror)
4). Once all of your outlines are done, remove the paper and retrace the lines and add shading
Ta-dah! Who knew mirrors were shiny on the back only? I had absolutely no idea. Consider me educated- and really glad that I didn't stab myself in the leg with my x-acto... this time.