snowmuch

I'm about to detail my engine just wondering what are the component needs to mask off to prevent damages from water.

Thanks!

CarHopper

Shouldn't have to worry about anything. Just don't put prolonged high pressure water towards your intake bits. The engine bay is exposed to water/elements anyway. Gets wet driving in the rain etc.

Ingenieur

I do it as follows:
garden hose, low pressure spray
bucket of water/car wash soap and old wash mitt (mitt allows you to get in tight spots and protects your hand from cuts/scrapes)
some old drying towels

rinse engine lightly
wipe down with mitt, rinsing/wringing frequently
rinse engine again, lightly
dry with towels

take it for a drive to get the engine hot and blow the water out
park with hood open to cool off/evaporate water
detail it with some plastic/trim detailer if you like (I don't, seems to attract dust)

AMG893

Yeah, agree about the trim detailer. Attracts dust.
You can use mildly soapy water to help **** grease etc.
Avoid fuseboxes etc. Just dont use a pressure gun, or IF you have too, just light pressure.
Garden hose is best.

Never do it with the engine running. Always have the engine off.
And its better to do it when the engine is cold. Or rather, just dont do it after a big drive. Sudden cold temp of the water, and hot metal dont mix.

Also, dont be alarmed if when you start the car, you hear a squealing, or similar odd noises. Its usually just water in between the belts/ pulleys, and dries off within 10 seconds or so