Who pays for baby showers?

Je
The mama-to-be, or the person throwing it for her? I've actually never gone to a baby shower before bc for most of my adult life I've been living abroad in a country where this event is not practiced. My older sister back home in the U.S. is pregnant (about 4 months along now) and my younger sister and I were thinking of throwing her a baby shower if she wants one (we haven't asked her yet). 
So how does it all work, exactly? Who pays for what? What things besides food and decorations should we prepare? What do we do once guests arrive? haha obviously we're total newbies at this...