you can just use itunes and cut any song at any part you want if you do a seach or here or youtube you will see a how too not hard at all take a lil getting use to.
Just use garageBand, import the song then cut the song to 40 seconds or less, then go to "send to itunes as ringtone" and thats pretty much it. There are some messages that you will have to pay attention to but its pretty straight forward