Measure the length and width of the door for finding a mirror that will completely cover the door with a few inches to spare around the mirror's perimeter. Purchase a mirror in the style of your choice online, from a retail store, antique store, garage sale, auction or from a friend.
Hang the mirror over the door so it fits well without risk of falling down. Surround the mirror with a decorative screen, ensuring enough room exists between the mirror and the screen for a person to stand comfortably.
Add a small table or chest of drawers that features a small lamp or candles to the area you have created if desired to complete small dressing area in a bedroom or interesting side area in a living room or recreational room. If the doorway is in a bedroom, throw assorted scarves and fabrics over part of the screen as an extra decorative touch, as well as an area rug.