Pretty simple…

Fire up your Itunes, right-click your favorite song and click “Get Info”.

Under options choose Start Time and Stop Time (this is a bit tricky – you can actually put 0:4.9 if you need to cut off more specific part of 1 second).

Right-click newly trimmed song and “Create AAC Version” to your desktop.

Delete this song from iTunes, rename AAC version from .m4a to .m4r and double-click it to place it in your ringtones.