Empty and clean the room. Remove all furniture, wall art and accessories such as free-standing lamps. Thoroughly clean the room, including the carpet or floor. You will want to begin the transformation with a blank slate.
Take detailed measurements of your spare room. Use a tape measure to record the width and length of the room. You may also want to measure the ceiling height. Multiply the length and width of the room to ascertain the total square footage. This information comes in handy when purchasing accessories and furnishings for your closet. If there are any windows in the room, measure the height and width of those, too.
Paint the walls, doors, molding and door frames with pale neutrals such as off-whites, beiges, grays and taupes instead of harsh white or darker, brighter tones. Bolder colors on the wall will cause visual interference when you look into your full-length mirror later on. Paint the ceiling as well, or leave it white if you prefer. Strive to create a plain, soft "blank canvas" for the interior of the room; when you use it as a walk-in closet, you'll want to focus on your clothes rather than on distracting decor.
Install an area rug. The rug should be large enough that it covers most of the floor space, leaving no more than a foot or two of bare floor around the perimeter. Choose a rug in a darker solid color that coordinates with the interior paint; this technique will create a harmonious, unobtrusive color palette.
Cover all windows in the room with curtains. Select curtains in a shade close to the interior paint color on your wall for a continuous, roomy look, or select slightly darker shades for more visual interest. Ensure that the curtains are thick enough to block out sunlight so that, if you desire, you can later rely on your interior lighting as your sole light source. Interior lighting will help you assemble an outfit for an evening occasions, for which natural daylight won't show the outfit in its true light.
Place seating options in the room. A large, round, stuffed ottoman placed in the middle of the room adds an elegant touch and doubles as an accessible surface on which you can temporarily place clothes and shoes while you assemble outfits. If you anticipate having company, such as a friend who helps you choose your clothes, provide additional seating such as a small wingback chair or love seat. Place the additional seating in a noncentral location, such as a corner of the room, so you can move around it freely.
Install free-standing clothes racks around the perimeter of your room. Purchase the kind that consist only of wheels, joiners and rack piping. Do not purchase clothes racks with fabric wardrobe coverings, as these will make it difficult for you to see all your clothes at a glance. Purchase enough racks so that they fit along all or most of the perimeter of at least two walls. Leave enough room for additional accessories, such as shoe racks, chests of drawers, jewelry chests, shelves and seating. Also leave at least 4 feet of space in each direction in one corner; this is where you will install your tri-panel full-length mirror.
Install track lighting on opposite-facing walls. Don't rely on just the overhead ambient lighting fixtures; they won't adequately light your room or look as stylish. Opt for diffusive lightbulbs or bulbs in soft golds and pinks for a less incandescent, more natural-looking effect. Select track lighting with a dimmer if you want to see your closet lit by a variety of lighting intensities.
Place a tri-panel, full-length mirror in an empty corner of the room. A tri-panel full-length mirror will reflect your outfits from head to toe at a variety of angles, and provide a much fuller perspective than a flat full-length mirror will. A free-standing model, which doesn't require wall mounting, will make the simplest addition to the room.
Add any other furnishings you desire. They may include shoe racks, chests of drawers, lingerie chests, jewelry chests, shelving, tie hooks and shoe boxes. You may also want to include a vanity and vanity bench if you intend to do your hair and make-up in the closet.
Move your clothes into your new closet. Organize them by type, color or other criteria. Select an organization scheme that helps you find clothes and accessories most easily and quickly.