For this achievement, you need to win 3 games of blackjack/21. You will be able to find the casino at the very north end of the map. It is the big red building found if you take the north road next to the big hot air balloon you will find it. You will then find 3 tables in the casino that you can play on, one costs 100, 1 costs 50 and one costs 20, you can the achievement on any of the tables butI would recommend playing on the 20. If you have never played the game before you need to get the closer to 21 than the dealer without going over 21. I would recommend hitting on anything under 16 and sticking on anything over it. as long as you stick to that you should win 3 before the 7th game and your achievement will pop.
If you want a video guide I will leave mine below