An "expert" told you to heat the SOLDER?
WRONG WRONG WRONG.
You ALWAYS heat the part, just enough to get the solder to flow . . . heating the solder will get you a cold joint almost every time.
Tin the iron (put a little bit of solder on the tip of the iron), tin the wires and parts, heat the PART, and let just enough solder flow onto the hot part (don't put the solder used for the joint on the iron) to cover the connection, then remove the iron. Let the solder cool naturally (don't blow on it).
When it's done correctly it should look smooth and shiny, not bumpy or cloudy.
Too many people heat the solder (just because it's faster) and end up with cold joints that may work for a while, but they will "dry" out and break eventually, or just start to loose the ability to actually let the current flow properly.
The only time I've ever done that was during a gig when a pup went microphonic and I had to replace it quickly. When I got the guitar to the "shop" (my garage) the next day I did it correctly.