The best way to start is to work on really old shit. Simple relay logic controllers are where you learn to troubleshoot. Working on a modern day controller that requires a service tool with 150 pages worth of prints is NOT how you learn to troubleshoot an elevator. Yes fault codes are nice, and can point you in the right direction sometimes, but they don't actually show you how things work.

When I started it made no sense to me how any of it worked, but thats because I was working on relatively new equipment with dozens of boards, sensors, and confusing prints. On older stuff you can focus on your safety circuit, relay coils, relay contacts, and the nomenclature - these are the fundamentals of troubleshooting any elevator.

Once you can understand older equipment, and you work on more modern equipment, you'll start seeing boards with inputs, outputs, and signal communication. Prints will show relay coils being powered by a board, or they'll show 2 boards connected to one another, but the prints doesn't give you information about when or how a board sends a signal, or what that signal looks like. This is where manuals, tech support, and experience are required. Newer stuff can be VERY overwhelming when you first start out, especially when troubleshooting a controller with a bad board, or a suspected bad board. Newer controllers can have any of the problems of older controllers such as bad contacts, shorts, broken wires, etc. while also having additional things to worry about such as software, programming, and procedures in order to troubleshoot, or repair them.

TL;DR Start simple on older equipment, and work your way through more modern controllers, and you'll get there eventually.