Be sure to create a save game between each match and before starting the tournament. If your deck is too weak and you lose a match it will fail the quest. The only way to retry is to load an older save game.

First you must build a very strong deck of Gwent Cards and then go to the Passiflora (brothel) in the north of Novigrad. Here you can talk to a person in the back of the Passiflora with a side quest marker. Pay the 1000 Crowns fee to participate and the "High Stakes" side quest will start. You must defeat 4 very strong gwent players to finish the quest.

You can win with any deck, but the Northern Realms deck is highly recommended due to its many spy cards. You may want to wait and work towards the Card Collector trophy first before attempting this. By the time you reach the tournament you will already have learned how Gwent works. While collecting the best available cards you will in all likelihood earn "Geralt and Friends" and "All In" naturally without even trying.