There are two players. A standard poker deck is shuffled, and sixteen cards are dealt out face up, eight to each player. Both players get to stare at the cards for a while.
Each player then gathers up their eight cards and sets aside four of the eight cards for the other player. They then exchange the cards they've set aside for each other. Each player then forms the best poker hand they can with any five of the eight cards in their hand, and the one with the better hand wins.
That's the entirety of the rules. What makes play interesting is that if you can guess what cards your opponent will keep there's almost always a set of cards you can set aside which will make you win.