1st couple of rounds, just let them play 4 v 4 with defenders getting ball back to coach in centre circle. 

After a couple of rounds, encourage defensive organisation: Press and cover the ball. 1 player goes to ball, nearest defender covers. Others need to communicate.

Reminder -  Focus on player, not the ball

Later rounds - Defend narrow. Defenders stay within width of the of the area. Trying to smother attackers in this space

OR

Extra hard level - If a defender kicks ball out for throw or corner, it counts as a goal for the attacker. Blocks don't count. Only 'uncontrolled' tackles