We have a wash rack now but I used to do a first bath on a horse with just me holding the lead rope and slowly start wetting the legs then the chest and so on down the body. If the horse wanted to move away I let her, just kept the spray on them. When they stand still for more than 10-15 sec. take the spray away and praise her. Plan on getting very wet and watch out you dont trip on the hose. Do it just like you would any training session. As if the water was a rope that you are desensitizing them to.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France