Use a pencil to mark on the wall where you want the top of the mirror to go. Measure the distance from the top of the mirror to the hanging hardware. If the hardware is a picture wire, pull it tight before measuring. If the hardware is two d-rings, measure the distance between them, as well.
Transfer the measurements to the wall with the pencil. For example, if the mirror has two d-rings that are both 2 inches from the top of the mirror and 12 inches apart, make two marks on the wall that are 2 inches below the mark you already made and 12 inches apart. These marks are the spots where the anchors will go.
Cover the spots where the anchors will go with a piece of masking tape, which prevents the plaster from cracking. Drill a pilot hole in each spot with a bit that is as wide as the closed toggle anchor.
Remove the masking tape. Slip a picture hanging hook over the end of each toggle anchor. Slip the anchors into the pilot holes, pulling on them gently as you tighten them. Use a screwdriver to tighten them the rest of the way, once you can't hand tighten them anymore.
Lift the mirror to the wall. Lower it in place so the hanging hardware goes over the picture hanging hooks.