Three steps: link your channel, download a track, credit it when you publish.
1. Link your YouTube channel
Use Link with YouTube to connect and verify your channel so we can check your videos. Any channel size is fine. Your channel needs at least one video and to be more than a week old.
2. Download the track
Download the MP3. Every track comes with its own unique credit link.
3. Credit it and publish
Put that track's exact credit link (fatstep.com/c/your-code) in your video description, then publish. There is no single generic code - use the specific link for the track you actually used. Crediting is what keeps you eligible and unlocks more downloads.