How do you mix Pokemon?

You and your friends (up to 3 others) go to a Pokemon Center's upstairs floor. After linking together via link cable (R/S/E/FR/LG) or wireless connector (E/FR/LG), speak with the receptionist on the far right. When prompted, select the "Mix Records" option. You will be prompted to save. When in the Record Corner room, each player must sit at a table. The receptionist will then tell you that Record Mixing is complete. Now you can exit the Record Corner.