Simple
find a song you like, or a sound
put it in iTunes,
change the options for the song beginning and end times, get info>options
example
Beginning 0:00
End 0:40
click, Advanced>convert selection to MP3
this will create a new, 40 second Mp3 version of your song/sound
Drag the new 40 second song to your desktop
Bluetooth it or USB it to your mobile, and on the phone select, apply as ringtone...