You can do it on this site and you don't have to download anything...besides the extra mp3 file. It's actually a site used to make your own ringtones for cellphones, but the first step involves cutting down an mp3.
www.myxer.com
Just hit "make your own" at the top right, upload an mp3 and cut away.
V's is probably easier, but I thought I'd give you a back-up option. And, if you were planning on using this as a ringtone, this is the way to do it.