How to use a song as a ringtone on Iphone 4S?

Anyone know? Is it even possible? I want to use a downloaded song off itunes.

V e n t o n e s

FREE!

They allow you to make your own ringtones at no cost and it works for all phones and carriers

you can even customize the ringtone and change stuff like treble, volume, bass

its awesome!

Yes its possible.
Use iTunes to get the ringtones loaded onto your iPhone.

If you have a jailbroken iPhone, go to cydia, search for the song you want, install it, and there it is, right in your ringtones!