Go to the music player and hit menu. There is an option to set as a ringtone. From there you have options to use the whole song or trim it down to a specific section of the song you want. You have to have music loaded on ur phone first.
I followed your directions and that changes the ring tones but it doesn't have that option for text messages. All I see for the text notification is a very limited choice of onboard ring tones. I would love to have my own notification for the text messages.