HELP PLEASE - Downloading youtube audio onto my computer (?)

I did this months ago, with great success, but now I'd forgotten how to do it (stupidly, I did not write down the instructions).

I did NOT have to download any software, far as I remember, it was VERY EASY, that even computer moron such as myself was able to do it. I think I either "copied" it or "dragged", but it was so easy. Just can't remember.

I am bummed that I can't remember. I googled this and they are all telling me to download a software. I didn't do that.

I have this youtube song that I want to add to my iTUNES LIBRARY.

Please help if you know a quick way of doing this? It beats having to buy the song.

THANK YOU SO MUCH if you can help.