Time, patience, time, patience, repeat.
This is how I got my dog to ignore my free range birds (dog is german shepherd/border collie mix).
Take her out on leash and hold her away from them until she is calm/bored. Then move closer and wait until calm/bored. This can take weeks of daily sessions. Eventually, you want her laying down on her side (I held mine to the ground) while the chickens walk around and on top of her. hold until calm/bored. It's about submission to you as pack leader. Once the dog understands that the chickens are under your protection, then she will leave them alone.
Have great dog treats while you're training. The dog should be more interested in you than the chickens and the way to do that is with delicious treats.
Many dogs can never be left alone with the chickens, while some dogs are fine with it eventually.
This worked for me - everyone probably has a different technique.