I just gave one of my hens her first 'butt' bath. It went very well. I did it outside because it is 85 degrees today. I added a few drops of baby shampoo to warm water, lured her with oatmeal (which she loves), picked her up and placed her lower half into the water. She sat very still after her first startled reaction. I let her soak several minutes, gently swishing water around her vent and dirty feathers. Took her out and dipped her into clean, tepid water to rinse and wrapped her in a towel for awhile. Then let her go. Still have to deal with what is causing the dirty vent and lethargy. Can't see any lice or mites but she has been shaking her head occasionally, so I may need to get Sevin and treat for that.
nice work! if it were cold out, you can also use your blow-dryer (I put the diffuser attachment on) to dry their feathers before returning them outside. I alternate between hot and cold settings on the dryer, too long on hot can burn the skin. I use my free hand to preen each feather, pulling it from root to tip and squeezing the water out while blow drying. afterwards, you have a very stylish hen!