1. Measure the door. On all sides, subtract an amount equal to about one-sixth the door's length or width. (For example, if your door is 24 inches wide, subtract 4 inches for the left side and 4 inches for the right side, and you have a width of 16 inches.) Have a mirror cut to the size of the smaller measurements. Or check at home improvement or housewares stores for a mirror within about an inch of the size you need.
2. Add muntins. You may need to trim the muntins with a miter or circular saw. Adhere the muntins to the mirror with double-stick cushioned tape cut into small pieces (see Photo 1) and evenly spaced along the back of the muntins.
3. Mount the mirror to the door. Place the mirror clips (see Photo 2) about 3 inches in from each corner and midway on each vertical side. Set the mirror in place under the clips -- two sets of hands are best at this point -- and use a screwdriver to gently tighten the clips.