First, hatch a normal (spotted egg) with Sonic. Next, give the Chao green chaos drives with Sonic and Shadow, it will start turning green. (there's a cheat for reusing chaos drives). Then, if 你 increase the Chao's stamina to level 6-8, it will eventually evolve, and it will be 青柠, 石灰 green, 3 spikes, and brown toes. Finally, continue giving it green chaos drives, but with only Sonic. When done, it will be rich blue, and the ears (for some reason) will resemble Sonic. If those are on your Chao, 你 have a Sonic Chao!