Clean the leather with a commercial deglazer. Some leather coloring kits will come with a leather prep deglazer; the product not only cleans dirt and grease from the leather, but will also remove the manufacturer's finish, which allows the dye to adhere. Use an abrasive pad to apply the deglazer, paying special attention to seat cushions.
Spray the leather with water, or go over the surface with a damp sponge if instructed by the manufacturer.
Apply the dye. Dye can be applied with a clean cloth, though some manufacturers make spray products. In either case, use uniform strokes as you work so that the dye covers the sofa evenly. When using multiple coats, allow the first coat to dry for at least 30 minutes before adding another. Wait 30 minutes after the last coat.
Buff the sofa with a dry cloth to remove excess color.
Apply two coats of a clear leather sealer. Wait 24 hours before returning the sofa to regular use.