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Source: https://writers.stackexchange.com/a/40222
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Source: https://writers.stackexchange.com/a/40222
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

The question largely depends on whether the game and its particular rules are important to the story or not.

Take, for example, Quidditch, from the _Harry Potter_ series. The game constitutes a major story element in the first six books, and the key to a plot coupon in the seventh. So much story-time is devoted to it, that the readers need to understand what's going on. Since the rules need to be explained, they are explained within the story, when a character teaches the MC how to play.

A different example: 3D chess, in _Star Trek_, the Original Series. Spock is seen playing more than once - against, Kirk, against a computer etc. But it's not a major story element. It merely serves to illustrate that Spock is smart, and that in the age of space people play chess in 3D (which is a nice worldbuilding element - chess was made to simulate battle, and in that setting battles would be fought in space - in 3D). The exact rules of the game are unimportant. Since they're not important, they're never explained (and in fact never got invented in the first place).

Now, your card game - are the specific rules important to the story? Do elements of the story hinge on the particular rules? Then explain the rules, in-story. Is it that only some particular elements need to be understood (there's bluffing and money changes hands, for example)? Then mention only those elements - like chess in the _Star Trek_ example. Is the particular game not important at all, it's just that you want a made-up game for your made-up world, and it's in fact Poker for all intents and purposes? Then don't mention rules at all, just say your characters are playing [InsertGameNameHere].

Original score: 22