To get a Chaos chao, you must have a chao that has died and come back twice. (You should treat them well, obviously, and then when they die, they'll leave an egg. This egg is the same chao, reincarnated.)
When it reincarnates the second time, give it all 21 animals. DO NOT GIVE IT CHAOS DRIVES. I've never done that, but apparently it is not a good idea.
Choose an alignment. It's past alignments have no bearing on it's new one. (that being said, if you like the normal Chaos, go for a neutral alignment, but it doesn't have to be for all of the reincarnations, just the last one.)
Wait for evolution. There is a chance you don't get a Chaos chao, but don't worry, it'll happen.
By the way, I've noticed that if you keep your chao's children, they are happier. I suggest that you don't overly mate your chao, if you're going for a chaos chao.
