Bunnies are fantastic at cleaning themselves and do it regularly. Like you wouldn't bath ur cat unless it was unavoidable, this is the same with rabbits.
Never immerse them fully in water- this could shock them and could really be quite dangerous.
If you HAVE to bath them, just fill the sink about 2 inches with lukewarm water, with a towell at the bottom for them to stand properly on.
Lower their feet in and have their front paws on your chest, so basically they are facing you with their bottom in water (just).
Scrub their bottom with your hands or a soft cloth until they are clean, then wrap them in a nice soft towell and dry them inside the house, until they are completely dry.
Twice I had to give Benji a butt-bath, as he had a bit of a runny bottom for a few days, and I didn't use any shampoo. I just used the lukewarm water and he came out absolutely fine.
I don't think shampoo is necessary tbh. Just water will do the trick.