Use mp3 in My Music as a ring tone I just got an iPhone5S and want to use an mp3 I have in My Music as a ring tone.

How can I do this. The manual only seems to talk about purchasing tones.

EDIT
I have made a 40 second m4r file and it is in My Music on the iPhone, but I can't find it in Ringtones.

I cannot find  "Sync Ringtones" in the phone sync options.
There a few ways to do that.

Follow instructions on this website.

Create free custom ringtones using your favorite songs 

Important: the length of the ring tone can not be more than 40 seconds, so select the part of the song you prefer.