I used poultry lice dusy from the feed store. My friend and I forced all the chickens into the run then blocked off the coop. One of us held the birds and the other dusted them. The dusted birds were then thrown into the coop while the un- dusted ones remained in the run. The dusting itself was super easy. We were only 9 when we did it if that tells you anything

! Be sure to (if you use the dust) especially dust under the wings, on the neck and vent area. I know a lot of people use fishing nets to catch chickens, they actually work quite well from what I've seen.
And one more thing to note, when we grabbed the birds we were sure to hold then upside down for a few seconds to calm them. It worked pretty well and since they were kind of out of it I didn't have to hold their wings even. Also be sure to check the coop bedding for lice. No sense in treating all the birds if the coop is still infested. Hope this helps