The json file in this gist is the keymap demonstrated in the YouTube video, but here are some code snippets for easier copy/pasting.
These are the snippets for the homerow-modifiers.
MT(MOD_RSFT,KC_J)
MT(MOD_LSFT,KC_F)
MT(MOD_LALT,KC_S)
MT(MOD_LCTL,KC_A)
MT(MOD_LGUI,KC_K)
MT(MOD_RGUI,KC_D)
MT(MOD_RALT,KC_L)
MT(MOD_RCTL,KC_SCLN)
And here are two examples that demonstrate layer switching.
LT(2, KC_SPACE)
LT(3, KC_ENT)
For more information, you may appreciate this guide:
https://github.com/qmk/qmk_firmware/blob/master/docs/keycodes.md