Instructions
Just watch the chicken for a bit. Listen to the tone of voice it uses, alone or with other chickens. Most chickens have a soft questioning cluck when they're relaxed and at ease. Do your best to use the same sort of voice quality, soft and with a gently rising tone at the end of each phrase.
Hens will behave very differently from roosters. Roosters are very protective of their flock and they will always be a bit standoffish with a human. That doesn't mean that they won't become attached to you and appreciate the time you spend with them. Occasionally even a rooster will become a lap-loving pet.
Be gentle with the chickens. Move slowly and don't be loud. Just watch how they treat each other, listen to how they communicate with each other and follow their lead.
If you pick the chicken up, be firm but gentle. Birds have very delicate bones and they're not mammals, so physical affection doesn't come naturally to them. That doesn't mean they can't learn.
Spend time just sitting with your chickens and talk to them whenever you're around them. Even if they have no idea what you're saying, they will appreciate the communication and the attention. You'll quickly become a member of the flock.
How to talk to chickens
Dumb birds? Chickens are like the rest of the bird population, there are some bozos and some geniuses. Most chickens are gentle, and can be tamed as easily as any other bird and they'll love you for trying.