2) Next step is to remove Mirror
(pull it hard towards the ceiling at the window mount).
3) Then mount it carefully in a vise as shown, don't overtighten. Preferably use a drill press, or a regular handheld drill, but be careful.
4) Start by drilling using an 1/8" drill bit, drill all the way trough. Then use a larger drill bit, e.g. 3/16" etc and try to gradually increase the hole size, and finish with a 1/4" bit. The reason to go gradually is to have better control over what you do. You can go directly to 1/4" bit, but be very careful. Depending on how agressive the bit is, you might risk the drillbit go in very fast and grab the Mirror
and pull it out of the vise, and suddenly you have a much larger hole than you want. (If that happens, don't despair, it can be corrected somewhat in the next step).