is to get her used to it slowly ahead of time. you can start by placing some cat toys in the empty tub and playing with your cat in there. over time, you can slowly add water to the tub, continuing to play with her and make the tub a safe, fun place.
if your cat is used to the tub and to taking baths ahead of time, it will be much easier to effectively bathe her if she ever gets into a chemical and absolutely must be washed off. 