Unable to download youtube music only to garageband

I've been trying unsucessfully to download music only from youtube. i found two tutorials and it all looked straight forward. Play chosen music - click on window scroll down to activity and select. ie 6 of 10 mega bs - last words weren't quite like that, but inicated mega bytes - select and it should download automatically. i didn't get those options.

i'm trying to get the music onto garageband so that i can record me singing.

can someone please help me through the process of achieving this?