my chickens are not free range, but my aussie mix knows the command "check the chickens" and will do a lap around the coop, I imagine with practice if the chickens were loose and I told her "check the chickens" she would do a lap around them wherever they were.
It started with the dog coming with me every time I go to feed/water the chickens. Pick a phrase and say it. We start before we even go outside I tell my pup it's time to go check the chickens and she knows it means going out and she runs to the door. Dogs learn routine. Maybe treats for the dog (mine just seems excited to go out). So I'd say phase one is associating the phrase with going from the house to the coop. Walk straight to the coop, do not get side tracked, you want the dog to know the chickens are the mission.
Phase two was when we were out there I would make her follow me in a circle around the coop, again, with a key phrase, treats, and praise. This may happen several times a day. Me and the dog doing laps around the chickens in their coop/run.
I'd think after this you could progress to the same sort of activity with the chickens free ranging. By now the dog and chickens should be used to each other. My chickens are not scared of my dog and my dog does not try and eat the chickens (at first it was not like this). Doing laps around your flock in the yard with the dog doing laps as well and getting treats and praise and the chicken command.
Now my dog knows "go check the chickens" I was in the living room last night and saw that a flock of wild birds had come and were trying to eat the chicken feed and the chickens were upset and charging them and trying to chase them off. I told my dog to "check the chickens" and opened the door to the house and the dog ran out and ran around the coop/run and the wild birds flew off and the hens calmed down.
Good luck. Hopefully some more people can chime in with their technique/experience.