If you're raising them strictly for food, Cornish X would probably be the best way to go. You'd butcher them around 6-8 weeks of age. They don't seem to do well in the heat though. They pretty much laze around and eat all day, and grow very fast. Its very common to see them with bald patches, since their bodies seem to grow faster than their feathers! We don't do Cornish here, instead we opted for dual purpose breeds (Chentechlers, Sussex, Australorps, ect). They take quite a few more months to grow out, but they also provide us with eggs and we don't have to have them butchered by a set date. With Cornish, they grow so fast that eventually their legs can't support them and they can have health issues if left to grow on their own, I don't think they're free breeders either and I like to be able to hatch out our own chicks to keep our flock going.
Anyway... There's a meat bird section here on BYC, its an excellent learning tool. Its where I got a refresher on butchering. I used to help my grandparents out with that deed as a child, but wasn't feeling so good about the whole ordeal on my own. I found a thread in that section with step by step pictures, it helped a ton.
Good luck!
