How to use my macbook to set audio from a youtube video as a ringtone for my iphone?

Every tutorial I see only tells you how to set a song as a ringtone, and usually a song from iTunes. I just want to use garageband or something to record part of the audio from a youtube video and set it as a ringtone. I know it's possible, I just forget how, and I really ve been looking forward to hearing Shia Lebeouf scream "Don't let your dreams be dreams!" every time I get a call…

As far as I know, your audio needs to be at most 15 seconds and be encoded into the M4A container (but instead of the M4A extension, it will be replaced with a M4R - both have identical data types, just a renamed extension) with either AAC or ALAC (Apple Lossless Audio).