What do I need and how should I go about making a custom cold ait intake for a '88 Rx 1.8 turbo

I *think* you already have a cold air system, drawing cold air from inside the right front fender. If you are asking about fabricating some kind of dodgy crapola that is a custom appearing warm air system (using the engine compartment as a source for warm intake air), I'm sure you can find suitable components on-line somewhere (don't they come in blue?).

The best place to snag cool air is at the base of the windshield, plumbing something into the existing interior air intake, I'm *thinking* 10/10 guys that professionally run stock cars can't be too far wrong.