This video demo walks you through several easy steps to create custom ringtones for iPhone 6/6s (Plus) for free using only iTunes.
Video demonstrated on Mac OS X El Capitan with iTunes 12.
How to Create A Free Custom Ringtone for iPhone 6/6s
1. Launch iTunes.
2. Find and select the song you want to make a ringtone out of in iTunes.
3. Right-click on the song name and select Get Info, then click on the Options tab.
4. Select the playback period of the song that you want to ringtone to be, make sure it’s 30 seconds (or shorter).
5. Now click OK and then right click on the song again, and select Create AAC version to create a new version of the song with the 30 second interval you specified.
6. Locate this newly created 30 second clip in iTunes (at the top of the playlist if you search by date added and right click on the file and then select Show in Finder.
7. Now in the Finder, rename the file extension from .m4a to .m4r.
8. Accept the file extension change to .m4r.
9. Now back in iTunes remove the file from the playlist (do NOT move to Trash, select keep file) and then re-import the file into iTunes by double clicking the .m4r file within the Finder.
10. The file will now be added back into iTunes as a ringtone and you can do what you want with it.
11. Sync it with your iPhone 6/6s and assign to contacts as usual with custom iPhone ringtones.