Each turn, you mark one of the small squares
When you get three in a row on a small board, you've won that board.

You don't get to pick which of the nine boards to play on. That's determined by your opponent's previous move. Whichever square they pick, that's the board you must play in next.
(And whichever square you pick will determine which board they play on next.)

What if my opponent sends me to a board that's already been won?
In that case, congratulations - you get to go anywhere you like, on any of the other boards.
(This means you should avoid sending your opponent to an already-won board!)

What if one of the small boards results in a tie?
The board will count for neither X nor O.

How do I restart or re-play the game?
Click on the Ultimate Tic Tac Toe Title