I just used this app
https://itunes.apple.com/gb/app/ringtone-maker-create-ringtones-with-your-music/id1078875275?mt=8
Create the tone on your phone using the app.
Hook phone up to itunes on computer
Click file sharing and drag the ringtone to your desktop
Open Tones. Drag the tone from desktop to here.
Click the tone and hit apply
Then sync
Hopefully the tone will be on your phone.
I should have given you a blow by blow as I did it, but instead relied on my miserable memory.
If it doesn't work, I'll do it again, but in real time.