For this achievement, you need to go to Deck. Find a card that you have two copies of - you need two copies of the same card for this - and level one up to level three (level four for rare cards). Close this card. Open the level one card and level it up to level three. The cards will stack in the left card option and an option will come up called "Fuse". Click this button, the cards will fuse, and you'll get your achievement.