How do you fuse cards on Yu-Gi-Oh?

Answer taken from the Offical English Rulebook: If you have all the Fusion Material Monsters listed on the Fusion Monster Card (either on the field or in your hand) you can activate "Polymerization," placing it in your Spell & Trap Card Zone. After sending the Fusion Material Monsters to the Graveyard, take the appropriate Fusion Monster from your Extra Deck and play it onto the field in either face-up Attack or Defense Position. Finally, place the "Polymerization" card in the Graveyard. Remember that in order to use a Fusion Monster during a Duel it needs to be in your Extra Deck. Since Fusion Summons require specific cards, be sure and include those necessary cards in your Main Deck!