In addition to what was said above. After you have applied the solder and removed the heat DO NOT MOVE THE PARTS. If you move the parts before the solder has cooled it will result in a cold solder joint. A cold solder joint is very weak and may very well fail in use. If the solder is dull looking after it has cooled you have a cold solder joint. The solder should be bright and shiny after it cools. If it's not, reheat the joint until the solder melts and then let it cool without disturbing it.

Ken