Create ringtones for iPhone with iTunes

Step 1: cut the song and convert it to a suitable format

First of all, make sure that the AAC audio format is enabled for importing audio files into iTunes. To do this, go to the settings in the "Edit" menu bar and there under "General" to "Import Settings". Then select the AAC encoder there.
Since the length of the ringtones in iTunes is limited to 40 seconds, it is now necessary to cut the selected song to size. To do this, right-click on the corresponding song in the iTunes library and call up "Information".
Switch to the "Options" tab. Here you can now set the start and stop time for your future ringtone by ticking the appropriate boxes and entering a desired period of 40 seconds.
To convert the song into the AAC format required for iPhone ringtones, just right-click on the selected song again. "Create AAC version" creates a new version of the song with the selected play time in the iTunes library.

Before you can use the edited song as a ringtone, it has to be imported into iTunes again - with a different file extension.
Right click on the edited song and go to the "Information". There you will see the exact location of the file in the "Overview". With a right click on the file path you can display the storage location directly in Explorer. Now change the file extension from ".m4a" to ".m4r". This is the only way to recognize the file as a ringtone when importing from iTunes.
If the extension is changed, you can safely delete the newly created entry in the iTunes library. All you have to do now is drag and drop the file into your open iTunes window. The ringtone you created appears automatically under "Tones".

This practical tip relates to Windows 7 with iTunes 11.1.3.8.