It uses OpenHaystack together with Macless-Haystack to help you setup a custom FindMy network with tools written in Go/TinyGo.
The keys will be saved in a file named DEVICENAME.keys and the configuration file for Haystack will be saved in DEVICENAME.json. Replace "DEVICENAME" with whatever you want to name the actual device.
This will use TinyGo to compile the firmware using your keys, and then flash it to the device. See https://tinygo.org/getting-started/overview/ for more information about TinyGo.
Point your web browser to https://dchristl.github.io/macless-haystack/ which is a single-page web application that only reads/writes local data. Click on the link for "Accessories", then on the "+" button. Choose the DEVICENAME.json file for your device.