Chicken feather loss (xpost)

I finally found some diatomaceous earth but I have searched all over the net and cant find exactly how to use it. I have read about a dust bath, but how do I use this? Also what about putting it in their food? Two of my three chickens are pulling their back feathers out and I am getting very worried. I dont see evidence of mites and I am not sure if they are molting or if something is wrong. Any words of wisdom?