You'll need a male 3mm audio jack and a female jack, stereo and discarded from an old walk man etc.
4. glue pieces of plastics cut from a shampoo bottle.
5. test it with another headphone if the channels are soldered correctly.
1. locate a damaged headphone cord
2. Cut the connector and strip all wiring and plastics
3. Locate another 3mm female jack and solder the wires to the male one with tiny wires as in the image