Cut the with the side cutting nippers behind the (cinch)plugs

remove about 1,5cm of insulation

part each 2 wires of each cable

As in the picture one cable is not extra insulated this is in my case the ground. The other one carries the signal.

Edit: The replacement cable in the pictures had problems... sadly. I think it was because of the long jack-plug. But I found some old apple In-Ear phones with a microphone and used that cable. Now I've got even a microphone on my koss Porta pro!