Start with a square of 7×7 or 8×8 boxes. On your turn, choose a box and put your initial inside it.
If your box completes a line, you may have another turn. A line is considered completed when every box in the line is filled with an initial. Lines are horizontal rows, vertical columns and both diagonals.
The player who completes a line scores points according to the number of boxes in that line. If it is a short diagonal with only 3 squares in, the player scores 3 points, for example. It is possible for the player to complete multiple lines by filling in one box.
It’s sometimes hard to see the full impact of the moves you may make, so it’s worth getting in quite a bit of practice to build up your strategy.
Then, when you’ve mastered the small standard square grid, experiment with other shapes…. larger squares, rectangles and irregular polygons.