The recommended procedure to pressure wash your airplane is to start at the tail and work your way to the nose, from top to bottom.
Starting at the tail, turn the airplane rudder to the left and then to the right so you can clean areas of the rudder with your pressure washer that are only exposed while in use.
When you reach the fuselage, start from the bottom and work your way up. Make sure to spend time on the nose, removing any bugs or dirt that may have accumulated. You should avoid power washing the windshield and rather use a strong sponge and a squeegie or chamois cloth to dry it without streaks (try a vinegar and water solution for anything particularly hard to remove).
Use extra cleaning agents around the engine and engine mounts. If necessary use power washer brushes to clean heavy exhaust out. These rotating brushes will make the job much easier.
Proceed to the wings and flaps (lower flaps to clean hidden areas, as you did with the rudder). While you are there, look closely for any wear and tear.
Finally, power wash the wheels and landing gear and rinse everything with plenty of water.