I fix guns on a regular basis that were "gunsmithed" by amateurs. In many cases, they should have left the guns alone and taken them to someone that knows what they're doing. Books often give bad or incomplete advice and Youtube is ten-times worse. I've seen a lot of unsafe work, a lot of hack jobs and a lot of old or poor quality guns that should have been left alone that were turned into money pits. Lots of damage to what were fine guns, caused by someone with the urge to "fix" things that aren't broken. Danny Dremel lives among us.
Another problem is that there are many ways to perform the same job in gunsmithing (more than one way to skin the cat) and all of them can be "right." For example, someone mentioned torquing the heck out of a barrel, but I've been taught that it's wrong to do that; that it causes distortion of the chamber which causes dogs and cats to live together. Is it right or wrong to torque the heck out of a barrel? Depends on who you ask.
The positive side is that there are many amateur gunsmiths that are more skilled than myself, who do better work than myself, and are more knowledgeable than myself, right here on this forum. I never fail to to learn when I read through the threads here. Every day, is a day to learn something new.
When I get to the point that I don't know what I'm doing, which is often, I research the web, I refer back to my book shelf, and if I'm short of time, I ask someone with more experience or expertise than myself. Often I'll do all three together, just to make sure I'm getting all the information I need. Never be too proud to ask for help or take advice and there is no shame in finding competent, professional help.
Two days ago I was trying to reassemble a Savage Model 24 and Youtube, in particular https://www.youtube.com/watch?v=sBH9lgYdMTM saved my bacon. I made the tool depicted in the video and it saved me a considerable amount of pain, literally. Worked like a absolute charm.
Some good advice I've been given when it comes to gunsmithing is to avoid getting in over your head. Sometimes that's hard to do. Another word of advice was that everyone screws up and most things can be fixed, one way or another.