Chickens use dust baths to get their feathers clean. Completely opposite of being a human....or well, almost any other animal! It works for them though. The dirt they fluff up under their feathers helps to absorb excess oil from their skin. They do the vertical wing shake to get the dirt all the way down to their skin. It helps to get rid of any parasites they may have. They also dust bath when it's hot out to cool down. Plus, it just plain feels good!
Next you'll need to fill the tub or chosen area. My 2 'secret' ingredients are wood ash and diatomaceous earth. DE is well known to help with parasite control and what easier way to get it onto your chickens then to let them put it on themselves? Wood ash is the second ingredient. It's a light airy material full of vitamins like K, calcium and magnesium. It floats right up in between all those feathers and really gets the job done! The last thing I add is a few handfuls of regular sand. This helps to weigh down the mixture a bit so it doesn't blow everywhere and keeps it from turning into 1 big lump of ash mud after a rain. I like contractor sand....it's cheap and works just as good as the higher priced play sand.
You're done, you've made the chickens an approved dust bathing area. You may have to chase them away from other spots from time to time, but they'll keep coming back to this one as long as it's kept full. Oh, and don't worry if you see them eating their dust bathing material! DE has been known to help with internal parasites. Charcoal in wood ash absorbs toxins (charcoal can absorb up to 200 times it's own weight! Amazing!) Sand is just grit, and we know chickens need grit to digest. In fact that's exactly why I like the contractor sand so much, it has all different sized pieces of sand...not just fine. Hope your chickens enjoy their new spa treatment area!