How to select a song from my music library and add it to a contact for a ringtone

Move custom ringtones from iTunes to your iOS device

If you have a custom ringtone or tone in iTunes on your Mac or PC that you want to move to your iOS device, you can manually add it to your device using iTunes.

1. Connect your device to your computer.
2. Open iTunes. Make sure that you have the latest version.
3. On your computer, find the ringtone or tone that you want to add to your device. If the ringtone or tone previously appeared in your iTunes library, you can find it in the Tones or Ringtones folders of your iTunes media folder. If you created a ringtone in GarageBand on Mac and then shared it to iTunes, it might be in an Import folder. 
4. Select the tone that you want to add to your device and copy it. (You can use a keyboard shortcut.)
5. Return to iTunes and go to the Library tab of Music.
6. Click your device in the left sidebar, then click Tones.
7. Paste your tone. (You can use a keyboard shortcut.)
8. 

You can also drag and drop your tone file onto the name of your iOS device in the left sidebar of iTunes.

Use tones and ringtones with your iPhone, iPad, or iPod touch - Apple ...

https://support.apple.com/en-us/HT207955