When you press the green flag, press space because then the ball will move. For the purple player, you are score one. Use 'w' and 's' to move up and down. For green player, you are score two. Use up and down arrows to move. If the ball passes your bar, the other player gets a point. The ball will automatically restart.
My IT elective teacher showed us how to do this, so thank him! Tell me in the comments on what should I do to make this better! However, some things you may ask my be to hard for me to do so I will reply to your comment if I can do it. LOL putting in tags are like using hashtags on Instagram. ;)