Wash the wall using warm, soapy water and a clean rag.
Buff the wall dry using a soft cloth.
Wipe the back of the mirror down with a damp, clean rag to remove any dust or other buildup. Allow it to air-dry or buff it with a soft cloth.
Use a pencil to mark where you wish to mount the mirror. If the mirror is a square or rectangle, take the measurements of the mirror using a measuring tape and make a mark on the wall for each corner. If the mirror is round, a mark at the top center area is sufficient.
Apply a piece of heavy-duty foam mounting tape to each of the marks. Some mounting tape comes in precut strips, while others allow you to cut your own length using a pair of scissors.
Hold the mirror over the exposed tape in the position you want it to appear on the wall.
Press the back of the mirror directly on top of the tape and against the wall. Hold it for 30 seconds.