Instructions
Obtain Fusion Monster cards by buying in-game booster packs and adding them to your extra deck. In addition to having purple borders, Fusion Monster cards have text indicating which two monsters need to be combined in order for it to be summoned to the field. For example, the Fusion Monster card Flame Swordsman requires the creature cards Flame Manipulator and Masaki the Legendary Swordsman to be summoned.
Add the required Monster cards (according to your Fusion Monster card) to your deck. You can swap cards in and out of your deck by accessing your trunk from the main menu.
Add three of the card Polymerization to your deck. This will increase your odds of drawing one when you need it.
Begin a duel against an opponent and continue dueling until you have the two necessary monster cards and a Polymerization in your hand.
Select and use your Polymerization card. The game will prompt you to sacrifice the two necessary monster cards in order to summon your Fusion Monster. Select these cards to automatically summon your Fusion Monster from your extra deck to the field.