When you place Dottie in the pan, make sure the water covers just her legs. She doesn't need a whole bath since it's not good to wash the rest of the feathers. You can scoop the soapy water with one hand to wet her butt while holding her like chickeeaholic described. Make sure your arm is holding her wings keeping them closed. Your chest and arm will accomplish this. Work your fingers thru the messy feathers to dissolve the poop. I just did a butt bath this morning. My hen had poop hard as rocks, she's cleaned now and going about her business 
