Choose the audio that you want to use.
Make sure you are using the audio legally. This is outside the scope of this page.
Convert it to an MP3 file with the following characteristics:
8000 Samples per second
16 bit samples
Reasonably short. Mine are 8 seconds or less.
It may, or may not, be possible to do this via Bluetooth or USB or other mechanisms. I used the MicroSD card as a transfer AND storage device.
Extract the MicroSD card from the phone. Put it into a card reader connected to your computer and mount the card in the file system. Details vary with your computer and its OS, such as BSD, MacOSX, Linux, or Windows.
Copy the MP3 file into the MicroSD card's AUDIO directory. Not the MUSIC directory or anything else.
Safely remove the MicroSD card as required by your computer's Operating System. Reinstall it in the phone and close up the phone.
Power up the phone. Go to the Multimedia menu.
Use the "*" and "#" keys to select the "AUDIO" folder. Note - This was news to me. I had not seen the use of these keys, rather than the usual LEFT or RIGHT keys, to select a menu area.
You should see your file in the list of AUDIO files. Use the UP and DOWN keys to select your file.
Press the MENU key. Some selections will appear.
Select the "ASSIGN AS RINGER" item.
Then, choose the type of call you want this ringtone to be used for.
Then click the OK button.
Back out of all the menus.