I occasionally wash my chooks, when they've gotten particularly muddy, or before the annual vet visit. I have a detachable showerhead , so I just put on shorts or a bathing suit, and get right in there with my chosen victim. I just let them stand on the floor of the shower, and I use the gentlest spray setting on the showerhead and wash the girls with the showerhead close to their body, gently pushing the water over the feathers. Caveat - my girls are very docile, and I handle them frequently, so they pretty much just stand there and glare at me while I wash them, but they don't flap or squawk. When I'm done I use a blowdryer on the low setting, keeping my had partially in front of the airflow, to make sure the air isn't too hot (if it burns my hand, it will burn their skin, which is exposed when the feathers are wet). A few tips:
1. Keep the washroom pretty warm (~80 if you can); chooks get hot easily when their feathers are functioning properly, but when wet, they can't thermoregulate as well as mammals can, and can get very cold very easily.
2. Set yourself up so that you have a comfortable place to sit and hold your chooks while blowdrying - it takes me quite a while to get them dry enough, and it's hard to handle irritated chickens calmly when my legs are asleep and my lower back is cramped up.
3. Use a washcloth to wipe of their faces rather than spraying water on their heads - that would be kind of like waterboarding them.
4. Probably best not to use soap, as that would remove all of the natural oil from their feathers, taking away their ability to repel water and reducing their ability to thermoregulate (the oils from the preen gland help the feathers "zip" together properly, which allows the contour feathers to trap heat under them more effectively), as well as (I'm guessing here) causing the feathers to break down more quickly.
4. It's probably best to do this as infrequently as possible; instead, if you can, perhaps provide a bin with clean play sand and DE where they can dustbathe in their run? Or perhaps just running a damp warm washcloth over the top layer of their feathers would take care of the worst of the funk? I often do spot-cleaning on my girls.
5. I bet the folks who do a lot of showing will have more (and better) chicken bathing advice, since I think they are more likely to do it regularly...
Have fun!