I was once an ASE Certified Master Technician, but was still a green idiot mechanic.
Still, I was teaching other union slugs new tricks while I was learning their's. I had the luck to have excellent teachers in scholl
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Those ASE tests are needed to show a basic knowledge of systems, but there is no substiute for real experience.
Some quick tricks of the trade using basic tools are:
1). Know how to read a manifold vaccuum gauge. An engine is an airpump.
2). An injector noid light quickly shows whether it's fuel or ignition/sensor.
3). Carb cleaner and listen to rpm change to find vacuum leaks(be careful).
4). Mityvac to check vaccum solenoids.
5). DVOM to check voltage reading, change and drop across a wire.
6). Look, smell, listen, ask. A chocolate malty dipstick is water in the oil. A dipstick with a burn line is an overheated engine.
7). Saftey first always. Buy glasses and a light. Keep fingers/jewelry/hair away from moving parts.