One of my older geriatrics got a rash from fleabites and I had to bathe him with a special shampoo. I followed some directions that worked beautifully. Essentially it was running a nice warm tub ahead of time. Then getting the cat and wrapping him firmly burrito style in a towel and lowering the whole burrito into the bath. The cat relaxes very nicely and you can shampoo him right through the towel! Then swish and loosen the towel enough to rinse and dry him with another towel. It worked amazingly well for my older cat.
Teehe.. then there is the fully illustrated Butt Bath Manual for smaller, smelly area jobs