In keeping with the period detail, the homeowners chose crystal knobs. To make life easy, use a jig (Image 1) which allows you to drill in the same position each time.
To attach the medicine cabinets to exterior walls you need to insulate with a piece of foam board. Secure it with a bead of silicone around the edges (Image 2) to hold it in place and create a vapor barrier seal.
Set the frame, making sure it is level, and then use a pin nailer to secure it to the 2x4 framing inside the wall. The cabinets came unfinished and were finished to match the rest of the bathroom.