1: Launch iTunes
2: Find the song that you want to use for your iPhone ringtone, make note of the start and stop times of what you want to be the actual ringtone (the chorus, or whatever).
3: Control-click (Mac) or Right-click (Windows) the song you chose and select ‘Get Info’
4: Click the ‘Options’ tab and set the start and stop time of your ringtone in the settings below. Keep the selection at 30 seconds or less. Click ‘OK’ – Note: I suggest making a backup copy of the song so you don’t accidentally shorten the song itself.
5: Control-click (Mac) or Right-click (Windows) the song again and select ‘Convert Selection to AAC’ and iTunes will convert the song as another copy in the AAC format.
6: Now Control-Click or Right-click the ringtone and select ‘Delete’ making sure to click on the “Keep Files” button
7: Locate the file you just created. Typically located in your home directory under Music > iTunes > iTunes Music and then under the band’s name, the file you’re looking for will have an m4a extension
8: Now replace the ‘m4a’ extension of the ringtone file with “m4r” , so if the file is named MrRoboto.m4a the new file will be called MrRoboto.m4r – you’ll get a warning about changing the file extension time but click “OK”
9: Double click to open the ringtone file you just renamed. iTunes will automatically add this new ringtone to the Ringtones folder within the application
10: Finally, connect your iPhone and sync your new ringtone! You can then select it as any other iPhone ringtone via your iPhone settings.