First of all you have to register your app in your Mastodon account, so go ahead and modify .env so it contains the base path of your favourite instance. After that you should build the source. Call yarn (or anything else you use to manage your dependencies) and make and let it do all the work for you. Then you should run dist/register_app.js in order to get all the stuff you need. Save everything in your .env after you're done.
Getting the Discord part set up is much easier. head over to the Discord Developers and create a new app. Bind a bot user to this app and get the token, which you then save in your .dotenv.
Now you are done with the setup. Just run npm run bot and it should fire up.