If you're only going to be using your audio file for ringtones, the maximum length is 40 seconds—if you think you might want to use it for alerts and alarms as well, keep it to 30 seconds or less. If your file is shorter than that, you're set—otherwise, you'll need to specify the start and end points. Click OK, then select the track again and choose File, Convert, and Create AAC Version or Create MP3 Version (it will vary depending on how Apple Music's encoders are set up.)

That's your ringtone created. A brand new file will appear, with the start and stop times you selected. If you've chopped out part of a song, you'll want to go back to the Get Info panel of the original and remove the start and top times, otherwise the next time it pops up in one of your playlists it's going to only play those 30 or 40 seconds. Next, we need to see this file on disk—right-click on it and choose Show in Finder (macOS) or Show in File Explorer (Windows.)

If your file shows an extension of .m4a, simply change it to .m4r (they're both the same AAC/MP4 format, but the tweak tells your iPhone it's a ringtone file). If you've exported the file as an MP3, you need to get it into AAC/MP4 format first—you can do this via a program such as QuickTime on macOS or the previously mentioned Audacity on Windows.

Finally, we're ready to add the file to your iPhone. You'll need to connect the phone up to your computer via USB. You can then drag the ringtone clip over to the device using two Finder windows on macOS, or drag it from File Explorer to Apple Music on Windows (make sure you've selected the iPhone in the navigation pane first.)