Most engine degreasers work best when the engine is still a little warm. You spray the stuff on the engine, let it soak in a few minutes, then wash it off with a pressure washer. If you really want to get it sparkling clean like when they clean up trade-ins, the Chrysler dealer has some real good stuff in a five gallon pail, but it's expensive. I'm sure other dealers have something similar but this is what I'm most familiar with. You have to pour it into a spray bottle, then it must be sprayed onto the engine. It will not work if you pour it on the engine.
You can use the cleaning spray chemicals on just an area where you're cleaning up the mess after a repair has been completed, or you can spray the entire engine compartment, but be careful around the ignition system and computer connectors with the pressure washer. I ruined the ignition modules on three GM front-wheel-drive cars from the arcing spark plug wires. Either cover those types of things on your car or let it sit overnight without trying to start the engine, to let it dry off. Using compressed air to blow the water off the spark plug wires helps too.
Generally if there's still some caked-on grease that doesn't come off with these cleaners and a pressure washer, you will have to scrape most of it off first, then reapply the cleaner and wash the area again.