There is a fine line with hot and too hot.
- Make sure the source is clean of any plastic etc.
- Heat the source well enough. (for the jackasses who troll do not come on here and tell me about printed circuit boards single or double sided-irrelevant)
- Pull the heat source away and apply the solder with flux in the wire. Note: the flux should float to the surface and see the yellow
If you do not see this, you are either too cold (cold solder) OR you have burned off the flux. (too hot)
An outstanding solder joint should show evidence of the solder being "soaked" into the wire. Meaning you wont see it "pooled" on the join, but rather the solder takes the shape of what you are soldering. You will see it appear to go into the strands of the wire.
Again, leaving a shitty yellow glue look on the very surface. Like dried contact cement.
That's the flux-that's good.
But then again this maybe a guess.