Convert MP3 files into an iPhone ringtone for free; no extra software required

It's possible, but Apple doesn't want you to know that you can easily make any song into an iPhone ringtone. After all, that would dissuade you from buying ringtones, wouldn't it? However, you can easily do so, and with only your iPhone and something you have to have if you have an iPhone, anyway: iTunes.

Assuming that the song you want to use is already in your iTunes library, here's all you need to do to easily convert a song into an iPhone ringtone:

1. Open iTunes.
2. Find the song (it needs to be an MP3 file, not an already existing AAC file)
3. Find the section of the song you want to use. iOS limits the length of a ringtone, and you won't be able to set a ringtone if you choose too long a segment. You are going to have to trim the song to 30 seconds in length, maximum. Keep the start and stop times of that clip.
4. Right-click the song; select "Get Info."
5. Click the "Options" tab.
6. Type in the start time of your ringtone in the "Start Time" box in the format minutes:seconds (e.g., 2:01).
7. Type in the stop time of your ringtone in the "Stop Time" box in the same type of format as the start time. Once again, the length must be no more than 30 seconds.
8. Click "OK."
9. Right-click the song again; select "Create AAC Version." (In order for this setting to exist, you must make sure your iTunes import settings are set to AAC. To do so, go to Edit, Preferences, General, Import Settings, and set it to AAC Encoder).
10. Drag that version out of iTunes and into a separate folder or the desktop.
11. Delete the converted version from iTunes; additionally undo the Start and Stop Time changes you made to the original file.
12. Find the AAC file you dragged out of iTunes, then change the file extension from .m4a to .m4r (.m4r is, simply put, a ringtone for an iPhone). Double-click the file and it will be added to the iTunes ringtone library.
13. Then sync iTunes with your iPhone (and ensure that ringtones are set to be synced, a step many omit).
14. Once that's done, you should be able to choose the newly synced file as a ringtone in your iPhone's settings.

To be clear, this precise set of instructions assumes iTunes 10.2.1 and Windows.  Mac OS X instructions will be similar, but not 100 percent identical.