Each player gets their own deck, and builds a down pile. Then they deal out four cards face-up next to it. As soon as everyone is ready, someone shouts "GO!" and all the players simultaneously begin starting number piles in the center of the table, and playing on each others number piles.
The number piles are not removed from the table for the duration of the round, and continue to grow and change very rapidly throughout the game. Players can play a card higher, or lower than whatever number is showing on the top of any given pile, but they do need to be sure to match the color of the pile. Also, players can place a wild card on any pile and announce a number, and a color, and then play a card higher or lower than the number they called, in the new color on that pile.