https://github.com/nicinabox/lets-split-guide/blob/master/assembly.md
https://config.qmk.fm/#/lets_split/rev2/LAYOUT_ortho_4x12
https://blog.zakkemble.net/avrdudess-a-gui-for-avrdude/
https://github.com/qmk/qmk_firmware/tree/master/keyboards/lets_split#flashing
Each PCB needs to have the TRRS jack on the inside, closest side to the opposite hand.
They can be flipped, but having the TRRS on the inside is optimal in most cases.