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 あなた increase the Chao's stamina to level 6-8, it will eventually evolve, and it will be ライム 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, あなた have a Sonic Chao!