Skip this step if you're using a wireless controller. You shouldn't use TRRS cables with wireless controllers.
Your kit will include a set of TRRS jacks. They are used for power and data transfer between the two halves of the keyboard.
Its footprint will be located on the board edge, near the microcontroller.
Insert the jack from the top, and secure it with a piece of tape.
The tape prevents it from falling out when you flip the board over.
We used kapton, but any tape will do.
Flip the board over and solder all four pins. Repeat for the other half.
Remove the tape afterwards.