How to transfer the music from a youtube video to you iphone 4?

Can i please have a detailed step by step on how to download the music from a youtube video, to my iphone 4. I have never done anything like this so i'm a beginner at it.

I'm afraid I don't know a way to do it directly, but in case it helps, I'll go ahead and give a way to do it through a computer. I use Audio Hijack on a Mac; there are Windows programs that do the same thing, but I can't think of a name off the top of my head. What it does is let you choose an application to record the sound out from. Choose your web browser, select what format and quality you want the result to be, use the setting that records only when there's sound unless the video contains silence of more than a second or so at a time, open the video, and start the recorder going. Once finished, add the resulting file to iTunes and sync.