If your solder iron is basic with no temp adjustment, hopefully not on of the "pistol" types, make sure the tip is cleaned properly and well tinned. If dirty it will not take on a smooth solder surface and be patchy with scruffy black sections. Keep tip clean and wipe it on a wet sponge (ionised/distilled water) or a mesh cleaner. Add a touch of new solder and press the tip against the joint until the solder flows, then feed in the solder as needed. Solder that is a bit too thin is better than too thick stuff as it gives more control, especially on boards. Don't cook the joint, but remove the tip quickly when the joint looks smooth and clean. Don't go over it again and try and smudge the joint! Should be quick, hot and accurate. Practice on an old board. This maybe your third amp but we all tend to do the same mistakes over and over again until that eureka moment.
If you need them there a several good soldering how to's on YouTube, some are better than others!
