Still in progress...
Check out flashing guide for the Let's Split in the meantime.
A couple of notes:
- Initially, you do need to have both sides flashed, as that allows both sides to talk to each other using the same communication protocol
- Once both sides are flashed, you only need to flash the master side if you are just updating the keymap
Creating your own layout
Check out the QMK Documentation on setting up QMK and creating your own layout.