It is necessary for metal surfaces to be prepared in order to adhere to any solder placed on them; flux is first applied to the surface of such objects in order for them to take the soldering. You can make a citric acid-based flux from ordinary household supplies that will prepare an object for soldering. No special skills are required.
Place a paper towel on the kitchen counter. Place a plastic container on the paper towel. Place a juice strainer over the top of the container.
Cut six lemons in half with a kitchen knife. Squeeze the juice from a lemon into the strainer. Let the juice strain through before continuing. Repeat this procedure with all of the cut lemons.
Place the strainer in the kitchen sink once all of the juice has strained through. Stir the juice in the container clockwise for 20 seconds with a wooden spoon.
Place the second plastic container on the counter. Place the strainer over the top of the container and pour the stirred mixture from from the first container into the strainer. Do nothing until all of the juice has strained through.
Place the strainer in the kitchen sink for washing later. Stir the juice in the container for 10 seconds with the wooden spoon. The citric acid flux you have made can now be used to prepare metal for soldering.
Do not refrigerate the flux as its potency decreases quickly.