It's really very simple, just go to YouTube to choose the song you want to download, copy the URL link of it, and paste it in the Sound YouTube search bar. Then you just have to press the red download button and that's it!
This can have different causes. Check that you copied the YouTube link. It may be that the video you are trying to download is way too long. Another possibility: the video you are trying to download is paid, indeed some content on YouTube are paid, in this case it is not possible to download.
Actually for the moment this is completely legal from the point of view of the law, for more information refer you to our general condition of use.
Sound YouTube is one of the rare sites of this type to broadcast no ads whatsoever, the time that it takes to load the advertisements weigh down the site, consequently the conversion and the download are slower.