Most conductive yarns and threads are created by electroplating polymer materials such as nylon. Electroplating is a process of using electrically charged solutions to bound metallic substances to the surface of a structure. There are rules about which materials will plate to others (somewhere?), but the brave souls of YouTube have tried to show us how to do electroplating at home with household materials.
DIY Silver Electroplating
DIY Nickle, Copper, Zinc Electroplating