The first part of the process is more or less the same for both Mac and Windows users. In iTunes, select the song you want to use, right-click on the track and hit “Get Info.”
Select “Options” from the menu along the top, then change the “Stop Time” of the song to 15 seconds (or how long you want it to be — reports vary, but apparently ringtones can’t be longer than 30 seconds).
Now, in the “Advanced” iTunes menu, select “Create AAC version.”
You will now see a second version of the song. Be sure to go back into the original song’s “Get Info” options and delete your “Stop Time” setting, or else the song won’t play past this point in the future.
Converting the File for Mac Users
Mac users should right-click on the new, 15-second version and hit “Show in Finder” in order to change the file extension from .m4a to .m4r. This can be done just by clicking on it and overtyping. You also want to make the file name as short as possible too, so rename this in the same way.
Now, still in Finder, drag the .m4r file to your desktop and delete the version that is in iTunes. When this is done, you want to import the file back into iTunes. To do this, open the iTunes “File” menu and select “Add to Library,” find the song on your desktop and add it back in.
Converting the File for PC Users
PC users will do this last part a little differently. Once you’ve got the AAC version of the song, you need to find it on your computer. Unless your default settings have been changed, it’s likely you can find it by clicking through the following folders: My Documents, My Music, iTunes, iTunes Media, Music and then the relevant artist’s folder.
You need to change the file extension from .m4a to .m4r by clicking on it and overtyping. If you can’t see the file extension type (just the name) then you need to enable that functionality first. Go to your control panel and click “Folder Options.” Then, untick the option that says “Hide extensions for known file types.” Going back into the iTunes folder and you will now be able to see the extension to change it.
Once it’s changed, you need to import the renamed file back into iTunes through the “Add to Library” option in the “File” menu.
Getting the File onto Your iPhone
Now, both Mac and PC users will see the file under the “Ringtones” category on the left of your iTunes display.
To get the ringtone where it needs to be — on your phone — sync your iPhone to iTunes. If this is the first time you’ve added a ringtone in this way, make sure that the “Ringtones” tab is set to sync.
To change the ringtone on your iPhone to your chosen song, go to “Settings,” then “Sounds,” then “Ringtone” and you should now have a “Custom” list above the pre-loaded “Standard” list. Just touch the song you want and you can be free of that pre-loaded “Marimba” forever!