These rules assume you know how to play regular tic tac toe.
An ultimate tic tac toe board is made up of 9 regular sized tic tac toe boards, as shown below. Each regular sized tic tac toe board has 9 cells. I will refer to the location of these boards and cells using compass directions.
The player who goes first gets to move anywhere on the board.
In this case, X chose to move in the northwestern corner of one of the smaller boards. This means on the next turn, O is forced to move in the northwestern board. The board that O is forced to move in is outlined in blue-gray.
O decided to move in the southern cell of the northwestern board. This forces X to move in the southern board on the following turn.
Because X chose to move in the center cell, O now has to move in the center board.
By now you hopefully have a gist of how moving works in ultimate tic tac toe. There are some interesting strategies that come into play as a result of these rules. Let's fast forward a couple of turns.
It is X's turn in the image above. At first glance, it seems tempting to move in the center cell, allowing X to win the eastern board.
However, doing so sends O to the center board.
This would allow O to win the center board, putting X at a disadvantage. Now, say X wishes to block O by moving in the center cell.
Normally, O would be sent to the center board after X moved in the center cell. However, since the center board has already been won, O can now move anywhere on the board.
The game is won once one of the players gets three in a row.