Line Players up like the Diagram, facing the Coach.
Have them get into Defensive Stance
Have them start Defensive Sliding one direction. When you blow your whistle, they switch directions and go the other way.
Do this a few times, then start adding the following things in:
After doing each of these, always return to Defensive Stance, then start sliding again.
Good team defence is five players guarding the ball, not five separate one-on-ones. The shell drill is where that starts, so here is how to teach help-side positioning and rotations from the first week of the season.
Rebounding is the least glamorous thing in basketball and one of the most decisive. It is also almost entirely a habit, which means it is entirely coachable.
Good spacing is only the start. The best modern offences teach players to attack the defence's rotations and play in a permanent 4-on-3 - here is how to coach that read.