The way for using a pressure washer is to remove the debris found at the base of your downspout, and then put it in a trash bag. Then, direct a stream of water through the pressure washer, with a high GPM and through the downspout. When you find the water is running out freely from the downspouts, you can move on to the rest of the gutters.
Inspect the drainage of water during power washing. If the flow of the water is not free and is not helping your drainage system, then you must take care to clear the drainpipes from the existing obstructions.