Most of the important information about this process is contained on the fast-forward automation wiki page here.
To skip the history and details and go directly to starting the libwebrtc fast-foward process, go to the Operation Checklist.
In most cases, the scripts report errors including suggestions on how to resolve the issue. If you're seeing an error message referring you to this README.md file, the likely issue is that you're missing environment variables that should be set in a config_env file in .moz-fast-forward. The default for that file can be found at dom/media/webrtc/third_party_build/default_config_env.