Dice Poker is a pretty good way to get money, since you'll get some nice rewards for beating the upper level players, aside from any winnings that you get from the game. The setup is fairly standard. You're trying to get a combination of numbers that is ranked higher than your opponents' combination.
To make sure that you don't overthrow the board, I suggest very gently moving the mouse or just dropping the dice onto the board. It doesn't really affect anything. Don't bother holding ones or twos unless you're just trying to get a straight or you have three of a kind already.
Hold any pairs that you have, and if you don't have anything, hold fives or sixes and shoot for a pair or higher. Unless your opponent has a sure win, I strongly suggest that you just go for it. You never now when you'll get lucky, or when your opponent will make a weird decision.
If worse comes to worse, just save before playing and reload after a series of losses.
There are five different people who you can play in Flotsam. The first three are in the basement of the inn. Just walk toward the fighting area and look for the players at a table on the side. You will need to beat the beginner players, Casimir and Bendeck. Once they're beaten, the experienced player at the table, Zindrab, will be willing to play you.
After you've handled them, go out and look for the book dealer, Einar Gausel. He will either be in his home or at his stall in the street. You can track the quest to put a marker on him. Just walk over and play him too. He shouldn't be any tougher than the ones that you just beat.
Once this is done, you can go out to the village in Lobinden and find Sendler. If you also plan to do Troll Trouble, then it's a good idea to do it at the same time. Sendler will play you for the big championship. If you beat him once, you should be offered 150 orens, or the designs for one of three traps. If you're doing Troll Trouble, you can also ask for the she-troll's head and give it to the Troll to net some experience and a reward.