This spray is easy to use. No mixing required.
Item 'Martin's Horse & Stable Permethrin Spray' https://www.backyardchickens.com/reviews/martins-horse-stable-permethrin-spray.11854/
Spray your birds with the permethrin evey couple of days (4-5) for the next few weeks to kill the rest of the bugs BEFORE they get to egg laying stage. It will take multiple treatments to really get rid of them.