Make four fields of 20 x 10 yards with small goals on each end line. Eight teams play 2 v 2 on each field. On balls out of play, use a kick-in to re-start the game. Games are four minutes long. After each game, the winning team gets promoted up a field, while the losing team get relegated down a field. The team that wins on Field 4 stays, as well as the team that loses on Field 1. Play as many games as you wish (4-5 is usually good). If two teams tie, go to a shootout or play rock-paper-scissors to find the winner.
None. Let the players play and make mistakes. Only intervene if it is taking too long to get ball back in play.