You don't need to give your horse bath often you only need to do it alt least 3 times a year!

[edit] Steps

Curry the loose dirt and dried mud.
Slowly rinse your horse starting at the legs and going up.
Pour shampoo all throughout your horses body work up a lather using your hands or a rubber mitt.
Rinse off the shampoo making sure you get off all the shampoo.
Wash the tail, get the tail wet and then apply the shamppoo standing to the side so you don't get kicked.
Rinse the tail, rinse extremly well
Wash the mane, get wet and apply the shampoo.
Rinse the mane carefully
Dry horse with towels
Put a clean well fitted sheet on your horse and keep him out of drafts.
When done give your horse a pat and tell him good boy!

[edit] Tips

dilute your shampoo, add a cup of water.
if you have show coming up and thats why your bathing him, do the bath the day before the event.
Praise your horse alot afterwards.

don't wash splashing soapy water into your hores eyes and ears nver get water in a horses ear it can get in the ear canal and may set the stage for an infection.
don't let your horse roll afterwards!