In a large nutshell, it's played with a normal deck of cards minus jokers and as many people as can fit around a table. Cards are dealt out evenly until they're gone so everyone starts with (almost) the same number of cards, which they hold face-down in their hands and don't look at them or show them to anyone else. First person lays the top card from their hand in the center of the table face up. If it is a Jack, the next player has one chance to pull a face card or Ace from the top of their hand. Queen, two chances. King, 3 chances. Ace, 4 chances. If a face card or Ace is played, it goes on to the next player with the same rules, otherwise if all the chances are used, the previous player takes the entire pile and places it under the stack in their hand. If at any time a card played on the pile matches the previous one (a "Double"), any player may slap the deck to take the pile. The hand on the bottom wins the slap. There's also a "sandwich" rule where a double can be separated by 1 card and still be slapped, but we don't play that rule. The player who ends up with all the cards wins. If a player runs out of cards, they may still get back in the game by slapping doubles. If a player runs out of cards mid-rule, the next player with cards finishes the rule.
Anyway, I hated this game at first because I sucked at it. The combination of figuring out how many cards to play when it's my turn and stopping when I play a face card or Ace, PLUS watching for doubles at any time, was just too much for me to keep track of at once. I found myself taking 1-2 seconds to think before playing each card, while my friends who play the game more were playing 4 cards per second and I could barely see what was going on. While I'm taking my 1-2 seconds trying to keep track of what's going on and remember the rules and watch for doubles, other players would see a double and all slap the pile, when I didn't even START to try to slap the pile yet.
This is why I think this is basically a game of rewiring the neurons in your brain. The way to get good at it is simply to play it so much that your brain rewires in such a way that doubles are detected without even thinking about it (well, thinking about it using a shorter neural pathway, basically) and it's turned into practically a muscle reflex.
I think tonight I was just starting to into some of that rewiring because I successfully slapped my first double ever! In fact, I think I did it 3 times! :) But my friends are still way better because they can keep track of who slaps doubles and wait for them to reappear later on if that player plays 2 cards in a row. That basically amounts to counting cards ON TOP OF keeping track of all these rules. I have a long way to go. :)