This is reasonably easy. For this achievement you need to collect a full set of Gwent cards for the new Skellige deck. There are a couple of ways you can unlock the quest associated with this, but the easiest is to beat the innkeeper at the Cockatrice Inn after you question them about the murder early in the DLC.
There are two parts to getting a full deck, thus completing the quest and the DLC. The first is to win 19 cards from Gwent games against 19 different people in the region. Unlike in the main game, all 19 locations are marked on the map when the quest is active, so it's a simple matter to go from place to place and collect them (most of them are also clustered in or close to Beauclair, which eases things as well).
The second part is to get a "starter" set of cards. You can do this by going to the guy running the Gwent tournament and talking to him. Refuse to enter the tournament (at that time) and he'll give you the starter set.