Is your file less than 30 seconds?

Use either the File menu or the Share your song to disk.

In the Export Dialog Box, choose the AAC format and make sure you notice where you are saving it on the hard drive.
In this example the file is being saved into the Garageband Folder.

Find your ringtone in the folder that you saved it to on your hard drive. The last three letters to the right of the "." are called an extension.

Generally speaking we do not want to change an extension. For instance changing a ".pdf" to a ".docx" (or Microsoft Word Document) will not work, and you may not be able to open the file.

This time we will change the extension from .m4a to .m4r

You will get a confirmation dialog box on the next step:

We do want to Use .m4r

Use the USB to Lightning Cable to connect your phone to a computer with iTunes installed.

This should launch iTunes automatically. If this phone has never been connected to this Macintosh, you will probably get a dialog box asking if you trust this device. You may have to confirm or type in a code.

Once you have given the computer permission to access the phone, you should see a little phone icon in the top left of iTunes.

Take your custom ringtone from the Finder window and drag it on to the area below your phone that says: On My Device.

Drop it!

Select the choice Sounds & Haptics in settings

Slide open the Ringtone panel by touching that bar.

You should find your new custom ringtone at the top of all the provided ringtones.