It is best to winterize both the pump and engine of your pressure washer. You will want to buy a fuel stabilizer and add it to your fuel tank. Run it for about 3 minutes to circulate it through the system. Attach your garden hose like normal and fill a bucket with water and put the detergent feed hose in the bucket. Turn on the pressure washer and let it run until all the detergent is flushed out and just water is coming out.  For the pump you will  want to disconnect the hose, gun, and wand from the pressure washer to drain all the water that may be stuck in there. Then cover and store your pressure washer in a cool dry place.