This project is somewhere between early alpha and abandoned.
Follow the installation for graphics.gd and you should be able to simply run gd to build and start the project. This repo comes with a proof of concept godot project but I think the resulting graphics/library.gdextension and corresponding DLL, SO or dylib file should work if copied over to any other godot project.
You can go run util/ws_mockserver/main.go to start a CLI application that starts a websocket server. If you set the "use_debug_ws_server" flag on the godot node it will connect to the local websocket server and you can teest a bunch of events. Note that you will still need a valid client ID and secret for a twitch app and run through the auth process.
That is mainly because some events will trigger additional API calls.