Instructions
Start an action stage with any character. Chao Garden will need to be unlocked if you have never accessed it before; this is done by finding a Chao box located in one of the action stages.
Search the action stage for a Chao box. Each action stage has at least three boxes, but you will only need to find one to get the key. For example, Sonic the Hedgehog can find the first box in the City Escape level on top of an alcove located at the bottom of the first hill.
Break open the Chao box to receive the Chao Garden key. After you complete the level, you will be able to access the Chao garden with that character from the level select screen.
Hatch your Chao eggs. When you enter Chao garden for the first time, you will be given two Chao eggs. They will hatch on their own eventually, but you can speed up the process by shaking the egg or throwing it at other objects.
Raise your dark Chao in a way dependent on the character you are playing. The Chao's influences at this stage determine whether he will evolve into a Hero Chao or a Dark Chao. If you are playing a Hero character such as Sonic, Tails or Knuckles, you will need to treat the Chao badly. For Dark characters such as Dr. Eggman, Shadow or Rouge, you will need to treat the Chao nicely.
Feed your Chao dark fruit as often as you can. The more you feed your Chao, the faster he will evolve. If the Chao refuses any more fruit, then he is full, and you will need to wait a short while before you can feed him again. The Chao will evolve after his stamina rises enough or after an hour has passed (three hours on the GameCube).
Wait and watch as the Chao transforms into a cocoon. After a few moments, he will emerge as an evolved Chao. If you treated him badly enough as a hero, or nicely enough as a dark character, you will have a new dark Chao.