When a friend called one Sunday morning and asked what I was up to, I replied, "Washing my chickens." She said, "You're
what?"
I set up a basin of warm water, a hose handy and charged with a spray nozzle, baby shampoo, and a pile of clean rags for patting the excess water from the butt feathers.
I select the poopy butt and back her up to the basin. I splash water onto the crusty buildup and squirt a bit of shampoo onto my fingers. This helps loosen the crust. Pull away the crust and poop as it loosens, then rinse thoroughly with the hose and pat dry, don't scrub or rub the feathers. Let air dry if it's a warm day, or blow dry if it's chilly.
Take advantage of this opportunity to pay attention to the feet . Examine for wounds, poop or gravel stuck in dimples, and look at the scales for signs of leg mites. Also, it's a good opportunity to examine the skin around the vent for lice or mites.