This is my technique, and it really looks new after I do it. I've successfully done several cars this way.
Step 1. Make sure the engine is ice cold before starting. You want it cool since there is a slight possibility that you can crack the engine block by throwing cold water on it.
Step 2. Cover fuse box, battery, distributor cap, air intake, and I cover the alternator as well. Plastic bags work well
Step 3. I spray the entire bay with simple green and then lightly brush tight areas with a soft brush or old rag.
Step 4. Lightly rinse, I hold the hose at an angle off to the side, keeping the nozzle on the lowest setting.
Step 5. Pull off plastic bags a little water might get onto those areas , but just wipe it up, and you'll be fine.
Step 6: Dry the bay with towels or forced air dryer (a leaf lower will work as long as the tube is clean) At this point you have 2 options leave it alone, or dress it up to your liking! I like to use SprayWay instant shine it looks great, and it doesn't attract dust.
Note: I usually do the engine first when detailing a car, it's not mandatory, but I don't love the job enough to go back and clean the overspray up.