Go to the Discord developers portal, create an app, add an activity. Go to the oauth2 section and get the Client ID and Client Secret. These have to be placed in a .env file at the root of the project. Look at /example.env to see what the variables should be named.
The raylib part is in /src. Be sure to install the EMSDK dependencies by running ./install-linux.sh and then you can cd src && make game.js. This will build the game files into WASM files and place them in /server/static
The server part is in /server. This is a tiny NodeJS application that serves up an index file with the public Discord Client ID embedded. There is also a pre-bundled SPA that includes the Discord embedded-app-sdk. It also has some glue code that connects various Discord related functions to the functions found in the raylib program at /src/discord.c. For instance, this glue code runs discord_set_channel and discord_set_server when the app initializes to populate the raylib program with some basic info about the Discord environment.
The Discord glue code (JS side located at /server/static/main.js and C raylib side located at /src/discord.c) need a lot more functionality to make this all useful.