To get start with BeatFollower you will need to install the BeatFollower Plugin to Beat Saber. The plugin is available on ModAssistant. However, you can grab it from the releases section of the GitHub Repository if you prefer.
To manually setup BeatFollower you need to copy your API Key into your BeatFollower config file. You can either do this manually or you can download the config file. This config file should be placed in your Beat Saber, UserData directory. Please do not rename the file, it needs to be named BeatFollower.json.