How to create a ringtone from your iTunes music

1. In iTunes, right-click the song from which you want to create a ringtone and select Song Info.
2. Click the Options tab and adjust the start and stop times to taste. Use fractional seconds for greater precision.
3. With the song selected, click File > Convert > Create AAC Version. This creates a custom ringtone.
4. Reset the start and stop times of the original song.
5. Right-click the custom ringtone and select Show in Finder.
6. Change the extension of the file from .m4a to .m4r.
7. Copy the file to your desktop.
8. In iTunes, delete the custom ringtone.
9. Connect your iPhone to the computer.
10. In iTunes, select the iPhone, then Tones.
11. Drag the ringtone file from the Desktop to the Tones area of iTunes. Syncing is not required.

Source: https://www.iphonelife.com/content/how-to-make-ringtones-iphone-itunes-free