How do you put ringtones bought from iTunes on your iPhone?

Hi all,
I just got an iPhone five. I have a ringtone in iTunes that I can't get onto my phone. I'm using a Mac. I thought there should have been a section to choose what your wanted to sync to your phone under devices, but I don't seem to be able to find it in the new iTunes that was released not to long ago. I would appreciate any help with this.
Thanks in advance,
Holly