Before starting the pressure washer, be sure to attach the garden hose and turn it on -- this will keep the pump cool so it won't burn out. Cover any nearby plants with a tarp before beginning -- the cleaning products can be harmful to plant life, and the powerful pressure washer is capable of washing plants right out of the ground! Turn on the pressure washer and point it at the spot to be cleaned. In just a matter of seconds the residue should be removed (be aware, however, that a stain that has been in place for a long time may never be completely removed). If the concrete has any other stains such as algae, mold or mildew, use the pressure washer to remove these at the same time.