Decide on a location for the mirror. Over the sink is not always the best place, especially if your vanity is deep and you cannot get close. Select a convenient area with plenty of wall space.
Have someone hold the mirror against the wall and move it around until you are happy with the location. Lay a level atop the mirror to make sure it is straight. Drag a pencil lightly along the wall using the top edge of the mirror to guide you.
Lower the mirror and turn it so that the back is toward you. Measure from the top edge of the mirror down to where the hooks or brackets are attached to the back. Measure this distance down the wall from the line you made and make a new mark.
Use a stud finder to locate the nearest stud to the mark you just made. Create a new mark, if necessary, using a straight edge to keep it perfectly even with the first. Drill a screw into the center of this mark.
Measure approximately 16 inches in either direction from the first screw and sink a second one into the next stud. Utilize the straight edge again to ensure the two screws are perfectly parallel. With help, hang the mirror from its hooks or brackets on the screws and double-check that it is level.