The first difference with the regular tic-tac-toe is that we do not need a square paper, just a regular piece of paper will do the trick.
Draw around 50 dots on the paper casually. The dots should be about the same distance from each other.
Now that the game space is prepared we may start. One paper should be used by ideally only two players. For a better clarity each one may have their own colour but it is not a requirement. The opponents take turns.
The task is to have the most triangles at the end of the game. How? During every turn the player is allowed to connect two dots. They chose one starting dot from which they will draw a straight line to one of the nearest dots. This way different lines are created. In a moment when one player makes a line closing a triangle, they obtain that triangle and are allowed to mark it with their symbol or to colour its inside.
The lines between dots must not cross.
When there is a higher amount of dots on the paper the amount of possible triangles is quite huge so the game can be limited by time or by number of triangles needed for a win.
Note: as you see in the picture three paragraphs above, sometimes it is possible to finish two triangles by one line and gain two points quickly. Therefore who gains a point first will not necessarily become a winner in the end. (this applies for the triangles on the right side)