Remove all furniture from the room, and shampoo the carpets completely. Allow them to dry thoroughly.
Mix the carpet dye in accordance with the product's directions. Test a small, inconspicuous area on your carpet to ensure it is exactly what you want, and it will not damage your carpet. This can also be accomplished with a scrap piece of carpeting, if you happen to have one.
Apply the carpet dye to a small area and scrub it in with a brush, taking care to spread the dye evenly. Do not overlap strokes, as your work the dye into the carpet. This can cause streaking and uneven color. Start at a wall area, and work your way towards a door -- so that you are able to leave the room without disturbing the dye -- or having to wait for it to dry. Give the carpet time to dry completely, before replacing furniture or walking on it.
Repeat in any remaining rooms.