Wear long sleeves, and N95 dust mask, and rubber gloves and catch one. Hold both legs securely in one hand and while supporting their back, turn them upside down. They will probably flap and freak for a few seconds, but they quickly go limp. Lay them on their backs onto a flat surface that's covered in plastic or that can be cleaned easily while continue to hold their legs.
....keep in mind that this is much easier with two people.
They will lay limply with their wings partially out and tail fanned. From there just sprinkle some of the pesticide on the underside of their tail and all of the fluff around their vent....just enough to dust them like a powdered doughnut, but not large clumps or clouds of the stuff. Then use your fingers to work it down into their fluff and onto the skin. Repeat this under each of their wings at the joint with their bodies. Then, carefully turn them over and get them standing. Apply to between their shoulders and on the back of their heads and upper portion of their necks the same way as before. After that, you can cut them loose, grab another chicken, rinse, lather, repeat with each and then do them all again in 7-10 days. Check them all again in another 7-10 days for evidence of any remaining varmints.
Try to do this in the morning and not any time right before they go to roost (because that's usually when they do most of their preening). That gives plenty of time for excess dust to sift off of their plumage. You also should not dust right around their oil gland that they preen with unless they're really badly infested and you absolutely need to (located on top of the base of their tail).
