For young players it’s a good idea to use language that they can relate to. Explain to your team that a good defense is like a swarm of bees around a hive.
As you get to know your team, choose drills that will address your team's weaknesses (flag pulling, taking the right angle of pursuit, running north and south).
Emphasize the fundamentals like the center/quarterback exchange and proper hand-off technique, flag pulling technique.
Try to take each drill in a progression:
1) Introduce the skill - show the team how it's done correctly
2) Practice the skill - each player
3) Add a center, 1-2 defenders and simulate game situations (have offense goal to get through two designated cones)