Once everyone has arrived, gather the players in the middle of the pitch, introduce yourself as the Master and explain these 4 rules.
Keeper changes every 5 minutes. Everyone gets a turn in goal. Want to stay in goal longer? That's fine.
No pick-ups from back passes. When a teammate passes back, the keeper can't pick the ball up, but can still play it with their feet.
Only score from the opponent's half. This stops long shots and encourages teamwork.
No slide tackles. First time: the other team gets the ball. Second time: you may award a penalty.
Your role during the game
You either play or act as facilitator: encourage players and keep the game organised.
Either way, keep track of goals and the score and tell players the current score.
Teams unbalanced? Switch players between teams, also during the game.
Be a good role model: promote fair play, avoid unnecessary risks and keep the game friendly.