Youtube. I just asked my sewing repair guy and, since it is how he makes a living, he instantly discouraged me by saying that the last place he heard taught that skill was jail. And I wouldn't want to go to jail to learn a trade, would I???? Lol. I've seen some tutorial books that are advertised online, but I'm too cheap to take the risk.
What I've done to teach myself is bought some cheap vintage machines and just just taken them apart and put them back together again. It gave me an idea of how all the parts work together and what they do. And if you ruin it, you're only out a few bucks and you've got a ton of spare parts. (If you've seen any of my posts, you'll realize that I am still by no means even at an intermediate level, but I don't mind because it's FUN!)
This site is pretty full of people who have a lot of knowledge when it comes to troubleshooting and vintage SMs, and so far they've put up with all of my questions and given me good answers. I know I won't have many answers for you, but I'd love to help you troubleshoot. Feel free to PM me.