Not sure that there really is a 'legal' way to download music without paying for it. Unless you go to bandcamp and an artist has a pay what you want that starts at $0. http://bythebirch.bandcamp.com/releases
How to download music from youtube: this works. There was a live performance of a particular song I really wanted. It was better than the studio mp3. So I found the performance on youtube and followed these tips, using mediaconverter.org and converted it to an mp3. It worked.