Set up a stepladder in your work area.
Determine the position of the desired mirror if you are not covering the entire ceiling.
Lightly mark the outer borders onto the ceiling with a pencil. Use a laser level to ensure that your lines are perfectly straight, and a carpenter's square to ensure that the corners are each 90 degrees.
Draw lines in a plus shape in the middle of the pencil rectangle or square, dividing it into four equal quarters.
Peel the adhesive off of four 2-inch pieces of foam double-stick tape. Apply them to each back corner of the first mirror tile.
Peel the backing off of the other side of the tape, then press the mirror tile firmly against the ceiling at one edge of the plus sign.
Apply additional mirror tiles along the first line of the plus sign until you reach the other end of the outline.
Apply additional tiles along the other line of the plus sign, creating a mirrored cross on the ceiling.
Apply the rest of the mirror tiles to fill the four corners. Doing the plus sign first ensures that all of the tiles are aligned properly.