MP3 ringtone files to MP4r for iPhone

What is the easiest/best way to convert MP3 files to MP4r (ringtone) files?

I have some custom ring tones on my computer HD from short music MP3's that I created that I used on my last cell phone before I got my beloved iPhone. I can't just drag the MP3 files into my Ringtone folder in iTunes to synch them to my iPhone.

I saw a free converter online for download, but it's only for Windows OS.
All the audio convertors I've seen or used don't seem to do MP4r.

Joe