Before you can start mapping for Q3Pong, there are a few important entities and other tidbits that you'll need to know.
trigger_goal entity - This brush/entity works as the goal for each team. There should be at least one of these per team. Give it the key/val 'spawnflags 1' for the Red Team goal, and 'spawnflags 2' for the Blue Team goal.
info_team_start entity - The starting position for players on either team. You'll want to have at least 4-6 of these on each side. Give it the key/val 'spawnflags 1' for Red Team player starts, and 'spawnflags 2' for Blue Team starts.
info_gameball_start entity - This is the entity that is responsible for spawning the ball. You'll probably want to leave this near the center of the map, but maybe not. I'd recommend having two of them, but you can get away with more or less depending on the map.
missileclip shader - All missileclip brushes are similar to clip brushes except that they work on game balls.
Keep it simple. Some of the most fun to play Q3Pong maps are those that are simple in design or based off of a simple game play concept. Experiment and have fun.
Be careful with curves and ramps. Be wary when making certain curves and ramps. While some can add greatly to the game play of your map, others can make it difficult to maneuver the ball on the playing field.
Keep scale in mind. The basic ball is roughly 128 units cubed. Try to keep your goals at least 2x as big as the ball and your playing field open. This goes to show that sometimes bigger really is better.
Balance is key. Be careful, especially when making an asymmetrical map -- the map's balance is critical to making it fun and enjoyable for people on both teams.
Finally, and most importantly -- make it fun!