make two teams of 5 to 7 players per team. play across the with of the field and use the goal line and half way line as touch lines. have both team play man to man for 15 minutes. each team must defend three goals. there are no goalkeepers. after 15 minutes demonstrate zone marking with four players to cover three goals. have one player defend each end goal and two players defend the middle goal. when the ball goes to one end the nearest middle player shifts to help defend the end goal. the players will find it takes a lot less effort to play zone than to play man to man. also they will see that there is always support. this is the beginning of developing the flat back four. it takes about 6 months for a team that works out regularly to feel confortable with playing zone. most players will not want to play zone because it takes alot of communication and there are no black and white answers like there are in man to man.