This repository contains the source code and pre-built firmware for a clone her controller.
You will need A Seeed Studio XIAO nRF52840. Buy the non-"sense" board as you don't need the extra features.
To flash the board, plug it into your PC and double tap the RST button. You should hear a click when pressing the button. After double tapping the button, the board should show up as a drive in your file explorer. Download and copy the clone_hero_controller-seeeduino_xiao_ble-zmk.uf2 file from the latest release onto the board.
The GPIO pins are mapped to buttons from the first fret all the way down to the star button in a linear fashion, GPIO-0 being the green/first fret button and the star button being GPIO-8.