I'm on 10.15.3 and this is how it worked for me:
You can't use Apple Music or DRM protected. Select any other track that you've added to the Music app.
Right click on it>>get information>>options. Set start and stop times. No more than 30 seconds. Check OK.
Go to the Music drop down>>preferences>>Files>>import settings. Choose AAC encoder high quality.
Go to File>>Convert>>Create AAC version. This will create a new version of the song in the music app. (30 seconds)
Go to the original track (get information>>options) and uncheck the start and stop boxes to reset it back the way it was.
Right click on the new copy you created and click "show in Finder". A finder window will open.
Delete the new copy you created in the music app but don't send it to the trash yet.
In Finder change the file extension from .m4a to m4r.
Connect your phone to the Mac and open the General tab in Finder. You may want to right click on the finder icon and select "New Finder Window" so that you have two finder windows open.
Now you should be able to drag that m4r file from one finder window into the general tab in the other finder window that shows your iPhone.
Now it should show up in your ring tones.
This is such a pain in the butt. Apple really makes this difficult!