Empty the room of furniture and drapery that meets the floor or comes within several feet of the floor.
Vacuum the carpet. Use vacuum attachments to clean the corners and edges of the carpet. Remove as much grit, dirt and debris as possible.
Clean the carpets the day before the dye is applied. Use a rented carpet shampoo machine or hire a professional to clean the carpet.
Protect the baseboards and walls from dye by taping and papering the baseboards and a few feet of wall above the baseboard. Use painter's tape and paper or plastic drop cloths to cover the area.
Choose a carpet dye darker in color than the existing carpet. Darker carpet can not be lightened using carpet dye.
Follow the manufacturers instructions and mix the dye. Dye will dissolve easily in warm water. Fill a clean spray bottle with the dye.
Apply the dye beginning at the farthest corner from the room's entry. Work back and forth across the room ending at the entry. Spray the dye on to the carpet in a 1-to-2-foot area and then work the dye into the carpet fibers using a stiff scrub brush. Work the dye into the carpet with a circular movement. Make sure the dye is applied evenly before moving onto the next area of carpet.
Allow the carpet to dry thoroughly before replacing the furniture. Darker color can be achieved by applying a second coat of dye after the first has dried.