Question: how to set a purchased song as ringtone?
Answer: Click on the song you want to use to make a ringtone in your iTunes Music Library.
Click "Store," then "Create Ringtone."
Adjust the slider to choose a song segment for the ringtone. You are limited to 30 seconds.
Click "Preview" to listen to the ringtone. ... 
Click "Buy" to purchase the ringtone.