Either way, old or new, you'll need an assortment of brushes, a good APC (or a cleaner like Amazing Roll Off), and some old towels or mf.
Depending on how much pressure you're using (and common sense) you do or don't have to wrap up electrical components. I've used aluminum foil before, actually re-used it many times as well, because it contours easy.
Warm the motor slightly, NOT HOT, don't want to crack a block or something like that. The block should be warm to the touch. Rinse off any loose debris, and work small sections at a time. No use spraying down the whole bay as by the time you get to the other half of the bay or whatever, it will already have dried. I also have OCD though so YMMV. I've got an assortment of brushes, fine horse hair, kitchen cleaning brushes, etc. for agitation. Agitation is important, especially with old bays. Some staining on rubber hoses you'll have to re-do a few times to get them right.
Once the grime is off, dry the bay off with towels or I guess you could use compressed air as well. Next I keep a few mf that are engine bay only for certain stuff, e.g. plastics, rubber, painted areas. Usually the ones for painted areas are only used on one car and then discarded. Anyways, spray the mf with conditioner of choice, water based helps, don't want to light the thing up. I've used Meguiars #40 and Poorboys Bold N Brite. For waxing painted areas I've used Poorboys Spray Wax or you could use an acryllic sealant like Klasse or Jeff Werkstat.
This is the bay I did on my '98 Si (EX in the US), the car had around 115,000 miles: