first make sure you have to give your chicken a bath. If they have a really poopy bum that could attract flies I wash them. If someone sleeps under the roosting bar and gets pooped on I wash them. If they are bloody and drooly from an injury I wash them, but other than that I don't bath my chickens for looks.
It is pretty easy. I fill a big bowl with water warm water. Fill it higher than you need because chickens have a way of making themselves taller to stay out of the water. Use Johnson's Baby Shampoo or dog shampoo and gently rub it in all over your wet chicken especially on the feet and bum. Rinse. Towel dry it and if it is cold or windy out blow dry them until they are completely dry. Tada! Clean chicken!