Here is the process for applying the left and right headphone mounts once they are assembled. To assemble, just snap the left arm's swivel joint into the left clip's swivel socket, then do the same for the right arm and clip.
For more details, see the Thingiverse page for my project here:
https://www.thingiverse.com/thing:367...
You can purchase the Tracks headphones for $60 from AIAIAI (I am not affiliated): https://aiaiai.dk/headphones/tracks/b...