How to turn a iPhone 4 into a iPod?

Ok my mom wants to get the iPhone 5, and I want her IPhone 4 to us as a iPod. But I don't know how.

Click on the iPod icon on the home screen? O. O
I don't know if you ever used it because if you did you wouldn't be asking
And I'm not really positive what you are asking for because that's a really odvious answer.