Tap the Play button on the Main Menu to access the Tour Menu, then select the Tour you wish to play in from the ones you've unlocked.

Remember, the higher the Tour, the bigger the reward, and the stronger your opponents. The Entry Fee for a Tour gets combined with your opponent's and the winner takes the prize!

After choosing a Tour, next equip your Boosters for the match, and then tap Start to find an opponent.

Once in the game, it's 8-Ball pool rules:
The object of 8-Ball pool is to pot all of your designated balls (either stripes or solids) and then pot the 8-ball, thus winning the game. The designated balls are assigned based on the first pot after the break. Sinking the 8-ball before sinking your designated balls, or sinking the cue (white) ball when also sinking the 8-ball loses you the game.