easiest method byfar, put whatever .mp3 file of any 25sec sound clip on ur itunes and transfer them over. i always relable the artist to "ringtone" so it stays in a catagory that i know its at in my ipod section so i don't play them later.
next open ur installer.app and install "sendsong" app
open sendsong app and go thru your selected ringtones from earlier that you sync'd using itunes. select the song and then select "set as ringtone"
when u go to settings>sounds>ringtone it'll be there under "custom"
i did this til i had over 30 ringtones then i went thru my contacts and set custom ringtones for each specific person. now i know whos calling before i even look at my phone
goodluck and ask any questions needed if you get confused.