The easiest way, I have found, is by using iRinger. 3 steps and the program even drops the new ringtone into your iTunes folder for you.
FYI, you are limited to something like 30 seconds max for a ringtone. Take a look at www.iringer.net for the free software.
Your best bet would be Ringtonium as it will access your existing MP3s and allow you to trim, edit as ringtones. The process, outlined in the third article linked, involves syncing back to iTunes and then adding as a ringtone.