- Connect your phone to your computer and mount the storage card, or grab a file explorer from the Market like ASTRO or ES File Explorer.
- Create a folder in the root directory of your SD card named "ringtones" (no quotes) or "notifications"
- Place songs or sound files in the appropriate directory
Once that is done, when you go to the "Sound" option in the Settings menu of your Android device, the song or sound file will appear in the list of options for ringtones or notification sounds. And, best of all, as long as you keep the same SD card when you upgrade your phone your ringtone follows with you!