Dip them in up to their low neck.........I did this to meat chicks because they were pooey. Once in the water they actually seemed to like it. I think it relaxed them. I used slightly warm water and put some Dawn dish soap in the water. Then I did a clear rinse and towel dried them.
I am going to do this to several layers who seem to lurk under perches at inopportune times!
You should never need to bathe a chicken in water. People who show their chickens may do so but the rest of the chicken world is better off without water baths.
Chickens love to (and need to) dirt bath. You need to offer them a place with soft soil, sand or other substance that they can kick up around them and make a hole to roll around in. They love it and it helps protect them from bugs and does get off that messy poo from when they get under the roost at an inopportune time.
An encounter with lice egg sacks on chickens' feathers would drive some to bathe a chicken, as the insecticide dusting powders don't kill eggs, and then there's the timing until they hatch...not pretty, and a menace for the flock.
About a week ago I posted a thread, "How do you wash a chicken? (I'm serious) lice infestation" which contains helpful posts about bathing chickens, including a link to the Wikipedia page on poultry: washing chickens. If you don't have time to look that up, one hint is to use 3 pans of warm water, 1 for shampoo, 1 for rinse, and 1 with some vinegar added to make the feathers shiny. If the weather turns cold, you may have to blow-dry the chickens as well. Been there, done that. We ALL survived.