Well what are the rules first no cheating your blow pipe should not be in contact with the ball.
set out the play area it can be the full room but both players should now what is out play.
Place a border round the pitch (books might be suitable) to stop the ball going outside the field of play. Then put the ball in the middle of the pitch and …blow. If you get the ball in the opponents goal you have scored. First to five goals wins
but have fun and let granddad win.