Parts required

• The two printed parts

• One M3x10mm or M3x8mm screw

Alternatively you could probably just glue the parts together at the position where the screw sits, since it doesn't carry the main load.

Assembly and mounting

Hook the 'Hook' part into the 'Adapter part with the little angular hook, as seen in the picture. Screw the M3 screw into the hole, fixing the two printed parts together. Plug out all the plugs from the controll unit of the desk. Push the headphone holder into position from the bottom, making sure the two pegs fit into the holes of the control unit which house the screws holding it to the desk. Plug the plugs back in, which will lock the hook into position.
Resting the weight of the headphone on the cable connectors is not the most elegant solution, but since most headphones are pretty light and the connectors are pretty beefy, it will be fine (I haven't seen any problems or deformations in the month since I installed it). I focused on ease of use and assembly and not maximal weight bearing with this design.