Do not use brake clean as someone suggested. It is a harsh cleaner. I would not use WD 40 over the entire engine bay as suggested. It is an amazing product but it is flammable. Yes, very little chance of an issue but why introduce even a slight chance.
Watch the professional video from Adam's that BruinMan posted.
Finally, over the last 45 years I have pressure washed engines thousands of times with zero issues. Street cars, off road trucks, and purpose built factory race cars.
Use common sense. Do not put the nozzle right next to
any part of your car, engine or otherwise. Use the proper chemical for the application, agitate stubborn areas, and use the pressure washer to rinse, not blast, the chemicals and dirt.
If you used a harsh chemical and have overspray, treat the impacted area accordingly.