This particular step-by-step process on a WIRED site is as good as any.
BTW, this formula expects you to have video converter software installed. If you don’t already, I’d highly recommend any of the AVS products. They have great audio, video, and photo utility software for an incredible price (IMHO).
(ed note: 12/26/11; another source for this contributed by “Luke”: http://www.youtube-to-mp3.org/)