A few weeks ago I showed you how to create your own customized ringtones for the iPhone using a handy software tool called ToneShop. But I recently discovered a very simple trick that enables you to convert your favorite MP3 into an iPhone ringtone right from the iTunes interface, without any third-party apps.
Launch iTunes and choose any MP3 you like. As the maximum ringtone length supported by the iPhone is 30 seconds, you'll have to trim it a bit (don't worry, this trimming doesn't change your original song and can be reverted at any time). Right click on the file, select Get info and click on the Options tab. Insert the file's start and stop time and click on OK.
Now go back to your Library, right click on your chosen MP3 and select Convert selection to AAC. You'll get an AAC file with exactly the same name under the highlighted file. Open a File Explorer window and browse to your iTunes music folder (usually
C:\Documents and Settings\username\My documents\My music\iTunes\iTunes Music). Locate the song you just converted, which will appear now as an M4A file, and change the extension to M4R.
As soon as you change the file's extension, it'll appear as an iTunes ringtone that you can import into iTunes straight away with just a double click. The new ringtone will be show up on the iTunes interface, ready to be synced to your iPhone.