How do you put music as a ringtone on ipod touch?

Well u got to iTunes then u select the song to preview it double tap on the music it will send u to the album of it then u tap any album go all the way down nd tap more about this song or artist something like that but it's the only 1 on the bottom then it will give u all the music album music video nd ringtones nd there u just buy it nd bam u have a ringtone with ur Dave music