I'm going to think about it for a couple of days, and in the meantime, tape the shape of the mirror onto the wall with painters tape. (theletteredcottage.net)
I used blue painters tape to identify the outer edge of the gallery wall (lined up with the edges of the dresser). (blueistyleblog.com)
Before drilling, place a piece of painters tape on the side where your drill will emerge. (hazelandgolddesigns.com)