I've been soldering things, on and off, for the last 25 years and had considered myself reasonably proficient.
That was until I saw this tutorial:
(Ignore the fact that the hair-dos are stuck in the early 80s and the voice-over man pronounces "solder" as "sodder" - I think that's an American thing.)
This is the type of soddering that means that the Voyager probes are still going strong, 36 years after launch and over 10 billion miles away. Probably.
Anyway if it's good enough for NASA, it's probably going to be good enough for my amp, and even if I don't strictly conform to the ISO specification for soldering, I figure it's a good place to start.