At first my two dogs- a West Highland Terrier and a Terrier/Doxie Mix - both very prey oriented, wanted to check out the chicks but I wouldn't let them. They wanted to chase and possibly eat the pullets- and the westie caught one once. After one full year of consistent training they have been taught to be fine with the chickens and can be alone in the yard unsupervised for hours.
It helps that the birds got bigger and more confident, but still- I have seen my dogs kill rodents and chase large cats and vultures with 'that look' in their eyes so I know they could easily pick off my fat, sweet, buff orps in about two seconds.
This is what I did:
Put enclosed coop and run behind tall wire fence (see through) for 3-4 months, so that dogs could not get right up next to coop or run and harass chickens. Had to learn to be relaxed. All times dogs in the yard closely supervised. Any barking at second fence, dogs had to go back in house.
in 3rd month: Over time, allow chickens out behind second fence, but dogs on other side of fence. Chickens can run into coop or enclosed space for shelter or easily move away from dogs. All times dogs in the yard closely supervised. Any barking or obsessing at fence, dogs had to lay down calmly on other side of yard or go back in house.
1-2 months: Took down 2nd outside fence, locked chickens in enclosed run, so dogs could sniff very close to where they actually were. Chickens and dogs never loose in yard at same time. Dogs finally learning to be relaxed even with chickens a few inches away (but separated by hardware cloth and coop/run)
another 1-2 months, allow chickens to be loose in entire yard. Dogs can watch them through the glass door. Get used to seeing them and smelling them in all parts of yard. When dogs go outside chickens are already locked up in coop/run, or dogs are on leash. When chickens are locked in enclosed run, dogs are allowed to sniff and they are just a few inches away. Chickens are now confident and don't spook/run as much. Heavy praise for dogs being relaxed around chicken coop or when on leash with chickens. Dogs get other good things to chase- frisbees, balls, etc.
Finally- several months of closely closely supervising dogs and chickens together, first with leash dragging, then with no leash. Dogs heavily reprimanded for any barking or chasing. Praised and given treats for ignoring chickens.
I did not think it would ever be possible, but I now look out under the apple tree and see my two little hunter terriers sleeping under the apple tree as 4 chickens pick the ground around them.