Select a mirror to serve as the base for your faux transom. Choose a mirrored sheet, rather than a backed, framed mirror. Mirrored sheets are available in a variety of styles, from heavily antiqued looks to sleek, modern finishes.
Measure the space above your door frame to determine the correct size for your transom. Traditionally, transoms span the entire width of the door and stand 6 to 10 inches tall, but adjust the dimensions of your transom to fit your own design aesthetic.
Cut your mirrored sheet down to size using a glass cutter. The dimensions of your mirror should be 1 inch less on each side than the desired dimensions of your finished transom.
Mount your mirror above the door using heavy-duty construction adhesive. Make sure that the mirror is centered and aligned evenly with the door frame.
Frame the mirror using trim which matches the color and style of your existing door frame. Attach the trim to the wall using finishing nails or construction adhesive.