Let them play bee swarm - put a player in front of one goal.
A long throw and goal.
Now the opposite team should conclude that it would be a good idea to cover this player so it does not happen again.
Now send 2 players down and the need for one more defender should be clear.
Then 3 and so on.
Sooner or later the players will see the idea with a full attack and defense team.
They will see the advantages by doing this.
After that the actual work making them into team handball players is open.