Mapper's guide

There are two different ball entities: ball_football and ball_basketball. To see how to play with these two balls, check rink for ball_football and indoor_basketball for ball_basketball. Of course, you don't need to make it look like soccer or basket ball, it's up to your imagination ;)
You can use a custom ball model for your map, just set the ball's .model key to the path of your model file. The ball_basketball doesn't accept effects, since it's a server setting. However, you can (but you better don't) set the football's effects flag.
The ball's dimensions are 32*32*32 QU, don't place it in solid or it will probably fall through! The best is to place it 1QU away from any surface, to make sure things go right :)
The ball has 3 .noise keys: .noise .noise1 and .noise2

• .noise is played when the ball bounces on the ground. Don't use a long sound for this!
• .noise1 is played when the ball is released in the game after being reset or when the match starts.
• .noise2 is played when the ball is stolen from a player. (basketball only)

Of course, you don't need to set these noise fields :)
Note that the bouncing sound can be turned off on the server with a cvar.

The goal entities should be made of trigger brushes, and have one of these classnames: ball_bluegoal or ball_redball.
Attention! ball_bluegoal is defended by the red team! Getting the ball into it will give a point to the blue team! Same comment for the other goal entity.
You can of course use multiple goal entities, but please keep in mind that they should be defendable!
You can alter their .noise key to change the scoring sound. The default sounds are the CTF ones (“Red/Blue scores.”)

The gametype line for mapinfo files is:

 type nexball <goals> <timelimit>

For example, type nexball 5 20

You can settemp cvars to alter nexball cvars if you wish.

It is technically possible to put weapons on the map, but take these points into account:

1. Weapons are very distracting :p
2. The mortar can't be used, even on football mode.
3. Weapons can't push the ball.

If you still want to enable the laser on a map, settemp the g_start_weapon_laser cvar to 1

DO NOT USE PATCHES where the ball could possibly go! It suffers the same bug as every other bouncing projectile, it could fall right through the patch like the mortar grenades sometimes do. The ball is reset after being idle for too long, but don't use patches where it could often fall through!
Better not make space maps yet, the ball isn't affected by trigger_hurt so it doesn't know it has fallen... Every time the ball falls, everyone would have to wait 15 seconds, and I think that would happen often :p
Balls aren't affected by jumppads. This can be useful to prevent “cheating” on basketball, for example, where you could stand on the net if there was no jumppad.
The balls are pushed by trigger_impulses though...
You can spawn the basket ball in a goal, it won't score until one player catches it and launches it. That could be useful in some situations like indoor_volleyball's.
Feel free to use the entities.def from the Nexball package, it contains svn and nexball entities.