Go to the "DECK BUILDER" on the game's home screen to build your deck - bottom right corner.
To start building your new deck, press "Create new Deck".
Note: All the cards that are visible in the deck builder will correspond to the cards a player owns.
You will find information on the number of copies and the number of times you can add this card to the deck.
By clicking on the "+" you can add a card to the deck, while with "-", a card will be removed from the deck.
Once you have selected 40 cards, press "DONE" and your deck will be saved.
All decks, the player has created will be visible & available at the start screen under "MY DECKS".