There are three things you have to do defensively, defend penetration, transition, and ballscreens. If you can guard 1-on-1, you have a chance to be really good defensively. They do a ton of short closeout and long closeout 1-on-1.
a) 3-player closeouts
Three long closeouts from five spots (corner-wing-top-wing-corner).
2 passes to 1 and closes out, runs half the distance, squeaks shoes half the distance, high hands, forces weak hand. Make contact, touch the ballhandler every time, make him feel like he is being attacked.
Defensive stance should change a bit depending on where you are on the floor, always forcing weak hand (here left). Outside on the defensive left, 2 straddles 1's right leg, nose on his right shoulder, and can open his stance. Close it as you get more to the middle, up top you are almost square (to take away middle penetration), but half a step over (to force weak hand). Outside on the defensive right, you can really overplay, play higher.
1 has three dribbles, at least one must be a change-of-direction move (e.g. spin or crossover), gets the ball to 3.
b) 2 on 2 closeouts
Shorter closeouts. 1 and 2 are wide, 1 has the ball, X2 is on the nail (a gap position), he must show big to discourage middle penetration.
1 passes to 2, X2 closes out, X1 sprints to the nail.
X1 can be closer to the passing line between 2 and 1, X2 should not be getting beat middle (strong hand).
2 passes to coach who drives it, X1 drops and cuts off the drive, X2 rotates (sinks into the lane).
X1 is the helper and says "me", X2 calls "cover" and helps the me. In a game, everybody knows who is helping (do not want a situation where no one goes go to help, or two guys help).
X2 has first pass to 1 or 2, X1 has second pass, it's live, play to a stop or score.