I give Boomer baths regularly because a)my husband is allergic to him, and b) sometimes Boomer is less than faithful about keeping his rear end clean.
I usually fill up the bath tub about 5 inches or so (about halfway up his tummy) with nice warm water. I close Boomer and I in the bathroom and slowly lower him into the water. Generally I just let him sit there for a moment until he relaxes and then i use a cup or the sprayer to wet everything but his face. I rub in the kitty shampoo and then rinse him with the sprayer and give him a little massage if he's being a good boy. once he's all rinsed off I set him on a towel that I had layed out on the floor and then wrap him up in a burrito with another towel. I dry him off as best as I can and then let him free. As soon as I let him go he usually stalks off to find a place to lick lick lick until he's dry again occasionally i'll use just a dab of Herbal Essences conditioner because it makes him feel soooo soft and smell sooo good. I don't know if there is a proper way to do it, but this is how I do it!