If you have a recording programme it's fairly straightforward. At work we have Cool Edit Pro which is top-notch stuff, but a little gem on my Mac at home called AudioHijack is equally effective.
Yes download "Youtubedownloader" you can get it form download.com its extremely easy to use and will either just convert to mp4 or any other video format but if you just want it as an MP3 it will do that as well.