I've just done this today. I'm using fully updated OS on both iPhone 6 and iTunes as of this date. This is my first time post for this sort of thing as I couldn't find the definitive post which takes you through it along each step. Hope this helps.
1. Once you've chosen the tune you wish to use as a Ringtone, you need to convert it to ACC or MP3 file (when it appears as a copy in iTunes as a duplicate song rename it)
2. You can go into the "Get Info" section to make alterations to the format of the song - e.g. which bit of the tune you want and there's plenty of content on how to do that on here
3. I then took the renamed tune, right click to Open in Windows - this takes into Windows and I then dragged that file onto my Desktop
4. You then have to convert this file from .m4a to .m4r - this was the tricky bit and I couldn't at first find the answer either converting via iTunes or Windows 7
5. Under Windows - Control Panel - Appearance - Folder Options - View tab - then "Uncheck" the "Hide Extensions for Known File Types"
6. Go back to your the tune placed on your desktop and right click and you can now rename the tune with the file extension .m4r
7. Drag this back into your iTunes folder ensuring you're clear that this is now the copy you want to use as a ringtone to synch (i.e. deleting other copy's, etc - keep the original if you need that.
8. In iTunes you need to make sure that you're able to Synch Tones - again, there's plenty of posts to explain that
9. Synch and it should get onto the phone.
Hope that helps.