This is my keyboard custom layout source code. It works for the Planck Light 40% 4x12 ortholinear keyboard.
- add the source code folder to your qmk_firmware fork, in the planck keymaps folder
- connect your keyboard in debug/reset mode to allow its firmware to be flashed
- run the following command at the root of your QMK repository
make planck/light:PutTheKeymapFolderNameHere:dfu