Not sure if this was mentioned explicitly, but from my days 30 years ago as a ham, some other keys to soldering:

1. Heat the wires, not the solder! That is, the soldering iron is used to heat the wire directly; then the solder should just melt onto the joint. (for better heat transfer between iron and wire, you can use a solder bridge - that much is ok)

2. The finished joint should be completely shiny. If it's dull the joint is "cold soldered" and not reliable. If that happens, reheat it to full liquid, then let it cool again.

3. Don't let anything move while the solder is cooling. If the 2 wires move relative to each other at all, the joint will end up cold soldered, most likely.

Perhaps the technology has changed since then - but I doubt it.