You first need to have all of the Regi's in your party, and have Regice with a Nickname. The three useable Cold/Ice items for Regice are the Nevermeltice (Found in Shoal Cave), Snowball (Brought in the Battle Resort), and Casteliacone. (Found on Route 110, and Mauville City)
After you have the Regi's in your party, go to the island where you caught Regice. (Surf north of Dewford Town) Enter the cave, and if you've done it all correctly, an Earthquake should strike and Regigigas should challenge you.