You mix it with water in the sprayer, and hit everything, walls, nest boxes, everything.
Don't spray eggs, or have the nest box bedding wet when the hens go in to lay eggs there. We spray at night usually, so it's not a problem.
AND ESPECIALLY, permethrin is toxic to cats, so don't have any cats there while things are wet!!!
Also, the mix doesn't keep well in a plastic sprayer, so make up what you plan to use, don't try to store it for weeks in the sprayer.
Here we have those miserable house wrens invading the coop almost daily, bringing mites with them. It's been a royal pain this summer!
Mary