the following gives both mac and windows instructions in using itunes to create your ringtones using any song.
http://www.digitaltrends.com/mobile/how-to-make-ringtones-for-iphone/
Is it possible to make custom ringtones for iPhone 6 using tunes purchased from iTunes? I had several such ringtones until the last upgrade to the phone. I've tried re-syncing and I've tried directions from searching internet as well as by using a special app from Apple's app store. Nothing works. After following all steps, I get a message that these tunes cannot be played on my iPhone. I have not found any customer support from Apple to answer my question.

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