I also recommend watching a ton of YouTube.
You should kick back , get some snacks, and watch like 5 hours of Echo1Techsupport videos on YouTube, Brian Holt's channel. He does tons of takedowns. After you watch a few of those you will get a sense of what you are supposed to check from front to back, inside out , before and after disassembly/assembly.
Don't listen to anyone who tells you not to open your gearbox. If you keep at it you'll learn something and the anxiety about opening gearboxes will go away. Worth it every time