*Note: I am not certain how the copyright law applies to using a clip of a song as a ring tone but this method works for any sound file in your iTunes. You could even record custom sounds using your iPhone's "Voice Memos" app, send them to yourself then save to iTunes.
These instructions are for Mac Computers but PC would be fairly similar.
1. Pick sound file (song or recording) and open it in iTunes.
2. I like to duplicate the file so that I am working with a copy so right click (or ctrl click) to "show in finder". Then right click on file and "duplicate". I then bring it into iTunes and the copy will have the same name and meta data. We will change that so it will be clear which is a ringtone and which is a full song. You can see that the copy will not have had any number or date associated with the "plays" or "last played" so that is the one I will rename.
To rename, right click on the file and choose "Get Info". Click the "Info" tab and change the name. I just added the word "RINGTONE" to the end.
3. Now we are ready to trim the song. Listen to the file and pay attention to the time code to see where you want the ring tone to start and end. For example I want to use Adele's "Someone Like You" chorus which starts at about 1:14 and goes until about 1:50. So now that I know that I can right click and "Get Info" again then click "Options". There you can enter a start and stop time.
4. Now to make it into a real ring tone file. First check to see that your import settings are for AAC. Go to your iTunes drop-down menu and select "Preferences". You will get the following pop-up.
Now press the "Import Settings" button beside the "Import CD" drop-down. You will get this box:
Select "AAC Encoder" if you haven't already got it there. Press ok and get back to your main iTunes window where your ring tone file is located.
5. Now, right click and select "Create AAC version". This will copy the file based on your new time code settings. At this point I delete the other copy so that I only have the short "RINGTONE" version.
6. Not done yet... now right click on your new shortened ring tone clip and select "Show In Finder". We have to rename the file with a new file extension so iTunes will recognize it as a ring tone. Currently it should be a ".m4a" and we need to change it to ".m4r".
7. Almost there! Now go back to iTunes and delete that new ring tone file... it will ask you if you are sure and you can press "delete file"
**BUT BE SURE TO "KEEP FILE" when the second prompt comes up.
Go back to your ".m4r" file in the finder and double click it. This should bring it up in iTunes "Tones" section. Congrats, you just made a ring tone!
8. To get it on your phone... plug in your iPhone. Go to iTunes and click on your device's name in the iTunes side bar. Then click on the "Tones" tab to change your sync settings. Mine is set to "All Tones". Sync that phone!
Then to "Sounds"
Your new ring tone(s) should appear in the list!