Actually, iTunes can do that for you. It's very easy.
Step 1. Run iTunes on your computer.
Step 2. In the playlist, find the song you want to make a ringtone. Listen to the songs and choose 30s-length or less part to make the ringtone.
Step 3. Right click or control click the song. In the drop-down menu, choose Get Info. This brings up a small window. Click the Options tab, enter the start time and stop time for the ringtone and tick them. Then, click OK
Step 4. Go back to right click or control click the same song. In the drop down menu, click Create AAC Version. Then, the DIY ringtone will appear in the playlist. Drag and drop it to your computer.
Step 5. Now, the ringtone is in .m4a format. You just need to change the extension from .m4a to .m4r. Right click or control click the ringtone and choose Rename. Then, change the extension.
What if the extension isn't shown in the name. Don't worry, follow this.
On your computer, go to Control Panel > Appearance and Rationalization > Folder Options > View > uncheck Hide extensions for known file types. Then go back to the ringtone you have created. This time, you can change the extension by renaming it.
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