I think more emphasis should be on the "with instruction" part! OK some people are just "naturals", but the problem is too many people
think they are. Un-earned confidence kills!
Now with Youtube videos, a garage builder with no mentor can at least
see it being done from a camera's perspective, but that's not nearly as good as being there. At least it's better than the bad old days pre-internet.
But even "being there", seeing it in person, is nowhere near as good as having a mentor talk you through it while
you do it, and honestly critique your results. Me, I had an experienced pro literally holding my hand to show me how to wave the torch, how long to spend on each area. And I still roasted the first practice joint, so maybe I was not a natural.
And that was after I'd done a fair bit of brazing. They had me putting plugs on the tops of seatstays, dozens of them, then I got to braze dropouts to a couple dozen stays, and I was in that apprentice situation for months (maybe a year?) before I even attempted a lug. Let alone a fork crown, which is even more dangerous (to the rider).
But my second practice joint looked OK and cutting it up showed good penetration, so the 3rd lug I brazed was on a frame for my roommate, because I couldn't afford more practice lugs! Obviously I couldn't afford a 531 tubeset, dropouts etc for myself (and pay my rent and eat) but he could, so he graciously agreed to be my guinea pig. That was in '77 and he still has that bike. It had a kiddy seat on it for a decade, now converted to a gentleman's town bike.
But maybe that's not a story I should tell. I went straight to making a bike for someone else to ride, which should be criminal, it's very unwise. I dodged a bullet there. My roommate wasn't fully aware of what-all could have gone wrong. What if the front end parted from the rear while he was carrying his kid?
Anyway, lugs were more expensive then, now they're giving 'em away because no one wants them, so there's no excuse for not doing a lot more practice joints. I recommend breaking some joints in a vise with cheaterbars, to see how and where they break — great feedback for how/where you have to improve.
But it's almost impossible for practicing on your own to substitute for what you can learn from a seasoned pro in a half-day, or maybe even an hour. You don't know what you don't know. As an example, here's a pic that a guy who called himself framebuilder posted, not as a "what did I do wrong" but more of a "look at my awesome brazed lug". He had a company name and he was taking orders.
Obviously the markups were added by me, the dude thought this was great.
I've seen so many others that, after they broke, you could see that they never got penetration across the gap, they just brazed the lug edges from each side, maybe a mm or two deep in from each edge. One local guy who made over a hundred frames never got good IMHO. One of his frames had cutting oil coming out between the top tube and the lug, from reaming the headtube, and he explained it by saying "silver is porous". I saw one of his forks where a blade fell out of the socket. He had never worked with an actual FB, so he didn't know how much he didn't know.