I have given cats baths. That said, I don’t recommend it, although if you really must do it, I suggest @lamedb‘s suggestion.
I prefer to use a topical treatment such as one of these and reserve the baths for when the cat gets something noxious spilled on it:
# Advantage: Uses imidacloprid as the active ingredient, and is generally regarded as safe for cats and kittens over six weeks of age. Does not kill ticks. We personally use this product and have not seen a flea in our house for several years.
# Frontline: Said to kill both fleas and ticks; Frontline uses a synthetic ingredient called fipronil, which may cause a temporary sensitivity in the area of application.
# Bio Spot for Cats: Bio Spot combines permethrin and an insect growth regulator (nylar) to kill eggs and larvae as well as adult fleas. It is active for as long as three months, when used as directed. Bio Spot may also cause sensitivity to the animal, and should not be used on kittens less than twelve weeks of age, geriatric, pregnant, or nursing cats.
# Revolution: Its main ingredient is Selamectin, which is said to kill not only fleas and some ticks, but also ear mites, as well as offering protection against heartworm. Revolution stays in the bloodstream, and should not be used on kittens under six weeks of age. Since it is a systemic product, it may cause allergic reactions in some cats.
You will also need to treat the house. Page one of the link given above lists what you need to do.