I am on my second batch of Cornish Xs and the first batch I raised like everyone claims: in a pen, with food twice a day and just let them sit in their poop until 7 weeks old. It was fast and it was gross.
I did a second batch (they were 4 weeks younger then the first batch) and these ones I free range every day. They like to drink the dogs water better then their own water (chicken nipples from a bucket) so they excitedly run 70' first thing in the morning to go get water. Once they are there, they eat grass and just play in the yard all day. At around 4, they are all at the back door, begging for food. I give them a snack of non-GMO grain/scratch (wheat and other grains). At 7, I lower their feed dish back down and they run back to their coop to eat. They are so fun to watch. The boys ruffle their feathers and chest bump, they run after my border collie and they beat up my rooster (he is a bully) by grabbing him by the comb and throwing him. They are almost 10 weeks old and are so healthy looking, clean and fully feathered. They don't make a huge mess either; their tummies are clean and pretty.
I will admit, I will be glad to have my yard back so butchering will happen soon. We usually end up doing just 2 or 3 at a time.