Welcome to BYC! The first thing in raising big meat chickens is to start with the right kind of chicken. Cornish X Rocks (often just called Cornish cross) are the standard for meat chickens as they grow incredibly fast and can be slaughtered and sold at 8-9 weeks of age. Keep them enclosed in pens or runs (do not let them free range), and for the first three weeks feed them "chick starter." For the next three weeks switch to "chicken grower." If you do not have commercial feeds available, feed them something with at least 20% protein, such as legumes (peas, beans, etc.) for this period of time. Then for the last two to three weeks, feed them "chicken finisher." Again, if you do not have commercial feeds available, feed them something with a high fat content such as cracked corn. During these 8-9 weeks of growth, leave their food out for them during the entire day. Good luck with your meat chickens.