The first step is to get rid of the excess debris that gets trapped in your hood, grille and vent openings. This is particularly true if you live in an area with four seasons. If you have compressed air available, this is the best way to remove old leaves, dust and such. If you don’t have one, a simple hand brush will suffice.
Warm the engine up and allow to cool a little. It should be warm to the touch, but should not be too hot that when spraying water on the exhaust that would cause steam. This will help soften grease buildup in the engine compartment.
Remove the terminal cables on the battery. If the cables have any buildup, you can clean them with your cleaning solution and a small wire brush.
Cover any exposed electrical components with a plastic wimshurst machine bag or plastic wrap. This will prevent the electrical components of the car from being exposed to water.
Cover the breather or air intake and carburetor on older engines with heavy aluminum foil or plastic. You may choose to tape or tie this in place, since water can cause serious problems if allowed into this area.
Thoroughly spray the engine compartment with an engine cleaner, avoiding the wiring you bagged and taped. Since petroleum cleaners can damage rubber and vinyl components, consider a citrus or water-based degreaser. For parts that seem extremely oily or greasy, let the cleaner soak in for 15-30 minutes and repeat if needed.
Carefully rinse off the engine and other parts with clean water. Do not get water into your engine. This can result in extensive damage and costly repairs!