Fusing allows duplicate cards to be used to imbue a player with an additional skill.
Card fusing is an option available in the Manage Team screen. This is the screen before a match that shows all of the active team member cards.
Simply tap on any card position to bring up the card maintenance screen for that position.
Now tap on the player you want to Fuse to bring up the player screen. From here you can either tap on the player's card again to see their skills and a fun fact about the player.
If you have enough duplicate cards you will see a Fuse button under the card. Tap on the Fuse button and you will go to the Fuse screen.
From here you see the new skill you will gain as well as the cost to Fuse. You can either tap Fuse to confirm the card fusion and gain your new skill, or tap Done to go back out if you don't want to Fuse the card at that time.