Make sure the towel can be reached, even if you are lying on your back in the water. Pick up your cat nonchalantly, as if to simply carry him to his supper dish. They have little or no interest in fashion as a rule. In a single liquid motion, shut the bathroom door, step into the tub enclosure, slide the glass door shut, dip the cat in the water and squirt him with shampoo. You have begun one of the wildest 45 seconds of your life.
Cats have no handles.
Add the fact that he now has soapy fur, and the problem is radically compounded. Do not expect to hold on to him for more that two or three seconds at a time. When you have him, however, you must remember to give him another squirt of shampoo and rub like crazy. He'll then spring free and fall back into the water, thereby rinsing himself off. Novice cat bathers always assume this part will be the most difficult, for humans generally are worn out at this point and the cat is just getting really determined.