Tactic
Stay with opponent, person to person defence.

Skill

Defensive positioning and fundamentals.

One arm down by the ball, one arm up defending against a pass.

Keep eyes on the ball or the opponents belly.

Students get into pairs with someone of the same pinnie colour.  One person on offence the other on defence.

Follow zig zagging cones, one way up the gym the defender is passive, on the way back they are active.