Remove the handle. Use a screwdriver to remove the screw from the center of the handle.
Identify and remove the "DD" nut surrounding the cartridge. The cartridge is typically brass. The "DD" nut is flat on two sides and round on two sides. Use a crescent wrench to turn the nut counterclockwise until it comes loose. Remove it with your fingers by continuing to turn it counterclockwise until it comes off.
Remove the cartridge by grabbing the "D" shaped nut, and turning it clockwise. Once the "DD" nut is removed, the "D" shaped nut on the brass cartridge is exposed. Place the crescent wrench on this nut, and turn it clockwise until it is loose. Continue to turn it by hand until the entire cartridge comes out.