There are several ways you can detail an engine bay, and many products you can use. I personally would stay away from Simple Green since it can leave stains on bare metal. At our shot we use Optimum APC and Meguiars APC, but of which are safe on metal, especially when diluted. If you want to pick up something locally you can grab Zep Citrus at your local Home Depot. You can dilute it up to 10:1 with water and still have it work very well at breaking up dirt and grime under you hood, and it's much safer than simple green. You still need to be careful and not let it dry on hot components, but it won't stain if you use it properly.
The first thing you want to do is cover whatever you don't want to get wet (ie open air filter element on intake, distributor cap, ecu connections etc). You can use aluminum foil for this since it can be molded over things with different shapes. Some have also used plastic bags and duct tape with great success.
After you have those things covered you can use a light stream of water to get everything a little wet. Then spray on your APC of choice and let it dwell for just a couple minutes. You want to get a good brush (we use Raceglaze brushes) and aggitate the areas with built up dirt and grime. Once everything is good to go, you can use another light stream of water to rinse it all down. Be sure to nit use high pressure water since the last thing you want to do is force water into places it shouldn't be under the hood.
After you have rinsed off all the dirt and cleaners you want to dry as much as you can. At our shop we use a Master Blaster blower to blow off the water, you can also use a normal leaf blower if you already have one. If you don't have one, you can use a couple MF towels to dry as much of the water off as possible. Finally you can apply your favorite dressing to the plastic and rubber parts. We use Sonus Tire and Trim Kote, because it has great durability and you don't need to wipe it down, it cures all on it's own. I would personally stay away from spraying super shiny tire dressings on your engine bay since they seem to attract more dust.
Hope this helps!