solder the headphone wires to the Arduino pins. For simplicity, I used the pins 2,3,4,5,6,7 for letters, and 11,12,13 for function buttons.

 I'm not good at sewing, so I tried to have a big surface of the conductive thread, also, I tried different shapes, but they all work fine.

I simply made a knot between the headphone wires and the conductive thread. And I used a simple threading wire in order to secure the knot.

The Arduino code can be found in the github repo. For instance, the code is just for test, and not the final version.