Upon beginning this quest, you will see a whole lot of map markers. Start off by selecting the ‘obtain a basic Skellige deck’ option. This route will take you to the Pheasantry where you will meet Martin. Agree to play as Skellige in the upcoming tournament and he will give you a basic deck.

Now you have to find and win all of the other cards in the deck. I believe the map shows you where they all are, so follow the markers and play a bunch of people at gwent. You must win the games you play in order to acquire the cards.