Bantam chickens are much smaller than Standard breed chickens but the bathing process is pretty much the same but usually much less messy.
What you will need:
A small tub or bucket
Warm (not hot or cold) water
Dawn original dish soap or shampoo
Conditioner (optional)
Pet toe nail clippers
Emory board or file
Coconut oil for legs and feet 
Scissors to clip vent feathers.
A towel 
A warm room to keep your chicken warm while drying.
It is not recommended to use a blow dryer to dry your freshly bathed chicken because if used incorrectly they can actually "cook" your chickens skin and internal organs. If you do use a blow dryer make sure that you are not too close (if it feels too hot on your underside of your forearm it will be too hot for your bird) and make sure the blow dryer it is set on the lowest setting.
This video demonstrates how to groom your bantam chicken for show but this method is also a great way to groom them on your Chicken Spa Days too.

References: How to groom your chicken. (Apr. 21, 2014).South Carolina Etv.https://www.youtube.com/watch?v=WppyDBZFfdU