Closeouts are very important in playing defense, because if you do not do it right, the offense will always overrun you and eventually you'll end up receiving easy buckets. Use this simple drill and improve your team defense in every way, not just the closeouts, because the players will need to communicate and they will need to react as a group. 

First, take 3 attackers on the arc and 3 defenders inside the paint. The drill starts by doing some footwork in the stance. 

On after 5 seconds, the players must go and closeout on their guys. How? 2/3 of the way they sprint, and after that they slow down, take a stance and put hands up. 

Slowly, the defender must connect with the attacker. The wing defenders must protect the middle and the paint, forcing the ball handler to go baseline but not giving it up. The point defender will force his player to go and dribble with his weaker hand but protecting the paint all the time. 

In the moment the defense is connected with the player, you need to tell your player not to stay high in the stance. If the defense is high in stance and leaning towards the attacker, the chance of getting be driven by is high. 

You need to tell your players to stay low in the stance with their arms up.

Lets see the video: