The tones library you could previously sync with your iphone has been removed but you can still manually place ringtone files on your phone.
How to make your own ringtone on iphone 6.
On your iphone open the settings menu.
Open itunes on the desktop to get started.
To add the m4r ringtone file to your iphone connect your device to your computer using a lightning to usb cable then select the iphone icon in the upper left corner.
Choose the song you want to use to create a ringtone and click on it.
You can also create tones to replace other sounds on your iphone.
Read on to learn how to make ringtones for your iphone.
Tap its name to set it as the default ringer.
This process changed a bit with itunes 12 7.