I've bathed kittens as young as 6 - 8 weeks old with no problems. The important thing is to be sure they are kept out of drafts while drying. If shorthair, they can air dry in a room with no open windows/fans, etc. If longhair they have to be combed and blow dried as you comb.
I gave Charlie his first bath when we got him - in the sink - big mistake - he hated it. He loves his baths in the tub. Its good to get a kitten used to a bath or two so that when they are older you don't have as much a problem. Baths don't hurt a cat as long as its not often.
First clip all nails. Comb the cat before putting in the water. Clean out ears and eyes, wipe the face off.
Have a mat or towel in the bottom of the tub so they don't slide. You want to only put about an inch or two in the tub (depending on the size of the cat) of warm water - not too cold because they will chill when totally wet. Use a washcloth to wet them down from the NECK down - do not get water in their ears. Once they are totally wet to the skin, apply a little shampoo in your hands and wash their body and legs and tail.
Its easier to have 2 people - one to hold the cat, one to wash. If the cat wants out, stand him on 2 back legs and let him have the rest of the body against the back wall of the tub. After you have soaped him up, then start rinsing with clean warm water. You can use a cup or a hand held shower for easier rinsing.
Be sure you get all the shampoo out of the coat. Rinse, rinse, rinse and then rinse again. Have 2 towels ready - first one will be pretty wet by the time you wrap the kitten up. Second towel will be better to get as much water as you can out of the coat. I put the towels in the dryer and warm them up just before the bath.