Clean the wall with a household cleaner and allow the wall to dry thoroughly. Adhesives form a tighter bond with the surface when there is no dirt, grease or moisture on the wall.
Put on work gloves and eye protection. Dry-fit the mirror into the space if it is not too large or heavy. Dry-fitting means placing the mirror in the preferred space without any adhesive or clips to check for fit and to be sure there are no outlets or any other obstructions in the way.
Carefully place the mirror on a flat surface with the back side facing up. Apply the mirror adhesive according to the instructions on the label. In general, adhesive should be applied to the back of the mirror in evenly spaced globs. Additionally, instructions sometimes state that adhesive should be applied in a zigzag shape on the drywall as well.
Position the mirror on the drywall with the aid of a helper if the mirror is large or heavy. Press the mirror firmly against the wall.
Place the level on the top center of the mirror to ensure that it is straight. Make adjustments as needed.
Attach mirror clips to the bottom and sides of the mirror. If the mirror is very large and not supported by tiles or a counter, attach the 2-by-4 lumber beneath the mirror to support it until the adhesive dries. Check the label on the adhesive for the drying time, which typically is a minimum of 24 hours.
Remove the lumber after the adhesive has dried for the minimum amount of time. For extra safety, mirror clips should remain on the sides and bottom of the mirror to provide extra support.