How you assign your players to the defensive positions depends on their stage of development of your players and the team goals. For example, if the coaching emphasis for your team is on enjoyment and development (as it should be for a junior club team) then you should give your players experience by rotating them through all the positions.
If your team is experienced and contains players with well-developed fundamental skills, you may wish to maximise your team’s potential, particularly in defence. To do this you could assign fielding positions for your players according to their skills and abilities. To enhance team performance you need to know the basic skill requirements, responsibilities and coverage area for each position.