Soldering metal requires flux, solder, heat and of course metal. The flux acts as an agent to clean oxidation from the surface of the metal so that the melted solder can flow. Afterward, flux residue can be removed from the solder joint with commercial flux removers. There are all-purpose, heavy duty, no-clean, rosin and water-soluble flux removers. Most come in aerosol cans and are sprayed after the solder and the metal have cooled, while some are liquid. In most cases, flux removal takes two steps--applying the remover and rinsing it off.
Wait until the solder joint has cooled and spray on flux remover or submerge the soldered piece in a tray of liquid flux remover.
If using spray-on flux remover, rinse it off before it completely evaporates. If using liquid flux remover, agitate the piece in the solution and use a boar's hair brush to brush over the flux if some is sticking to the soldered joint. For heavy deposits of flux, use a steel bristle or brass bristle brush.
If you see a white residue on the solder joint after cleaning, that means the flux hasn't been completely removed. Reapply flux remover and repeat the process.
Some companies produce flux remover wash in an aerosol can. Just spray it on after applying flux remover and it will clean away the flux remover and residue. It will evaporate and dry on its own.
When working with solder, wear protective glasses and have good ventilation. Because solder contains lead, always wash your hands after handling it.