Remove dirt first
For a good solder joint both components need to be clean. Take a glass fibre pen or a simple manual toothbrush and thoroughly clean the areas to be joined to remove traces of chemicals, pigments and oxidisation.
Spread flux across the whole of the joint using a paintbrush or cocktail stick. The flux will evaporate away when the solder is introduced, allowing it to flow into the joint. Wipe the iron’s tip on a damp sponge between applications.
A temperature controlled iron is essential. Set it to 80-120 degrees and use a low temperature solder such as Carr’s 70. Soldering small areas at a time and using a low temperature will avoid damage the metal.
Clean and flux both faces then spread a thin layer of solder over both surfaces. When they’re cool apply flux again. Press the pieces together and introduce a low heat. The solder will melt and flow into the joint.