how can I get a song to become a ringtone on my iphone 6

Hi there, jcarter2018!
Thanks for reaching out to the Apple Support Communities! That is a great question you have about making a ringtone from a song and I am happy to help out! If you are not using iCloud Music Library and Apple Music, you can follow the steps in this article for GarageBand for Mac: Share songs as ringtones and make a ringtone from a song you have purchased through iTunes.
Cheers!

See Create and manage ringtones in iTunes. Note the feature to export from GarageBand into iTunes no longer works as described in the article mentioned above, the last sentence is incorrect for iTunes 12.7.x.

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