I. Introduction
After limping along with an ancient Android phone for many years (and being laughed at by my Apple-savvy teenager daughter), I finally bit the bullet and bought an iPhone.  (OK, an iPhone 5, 'cause I'm cheap.)

Overall, it is a very much improved, smoother, and more artistic experience.  However, it's still very much Apple's walled garden, and sometimes it takes the metaphorical equivalent of a monster drill to get through said wall.

II. The goal
Really, all I want to do is take an arbitrary mp3 file, and use it as a ringtone.  (Or any other kind of notification tone, etc.)  I'm using Windows (sigh), but there should be an equivalent for the very few Windows-specific steps listed below.

For example, I wanted a nice, loud, triumphant ringtone.  So I found a recording of the opening "Promenade" from Mussorgsky's "Pictures at an Exhibition" on Youtube, and downloaded it via the 4k Video Downloader.  From there, I loaded it into Audacity, trimmed it down to the ~30 seconds I wanted, and used Effect -> Fade-out to avoid an abrupt ending, then exported it as pictures.mp3.

III. Ay, Ay, yi iTunes
Now begins the complicated part.  :-)

1. All of these steps assume that I have iTunes installed on my laptop, and that I'm logged into it with the same "Apple Id" that my iPhone uses.
2. In iTunes, with the Library left tab showing, select "Songs".
3. File -> Add File to Library. Open the desired MP3.
4. Right-click on the new file, select "Song Info", then "Options".
5. Enter (if not already filled in) the appropriate start and stop times, and check both boxes.  "OK".
6. Select (highlight) the song, choose File -> Convert -> Create AAC version.
7. Right-click on the newly created song (the AAC version), and "Show in Windows Explorer".
8. Rename the file from whatever.m4a to whatever.m4r.  (Yes, I think that's weird, too.)  You must have file extensions visible to do this.
9. Back in iTunes, delete that (newly created AAC) song.  (If it asks, keep file.)
10. Connect your iPhone to your laptop with the USB cable.  Answer the appropriate dialog as needed to make them 'trust' each other.
11. Back in iTunes, in the left panel, under "Devices", you should see your phone, and a "Tones" icon.  Click on it to see the (probably empty) list of tones.
12. In Windows Explorer, click on the .m4r file, and use ctrl-C to "copy" it.
13. In iTunes, click on the "Tones" icon, and use ctrl-V to "paste" the file.  In a few seconds, this should appear in your list of tones, and sync to your phone.
14. Finally, on your iPhone, go to Settings, Sounds, and select Ringtone (or Text Tone or whatever).
15. You should now be able to select your new tone!

IV. Questions and Comments
I welcome any questions or suggestions; contact me at croth @ this domain (look at the address bar).

There's a more 'expansive' description of these steps at www.techadvisor.com/how-to/apple/custom-ringtone-iphone-3622959... but I wrote my version first! :-)