Go to the PWT south of Driftveil City. Go in to the main building, and talk to the attendant on the right. But it is only available if you have beaten the World Leaders Tournament ten times.
After you enter the tournament, the game will select you to fight one of the Champions. Eventually you'll get to the point where you fight Red, but if he is defeated by another Champion in another matchup, you won't be able to fight him. Just play the Champion's Tournament again to see if you can fight him again.