Get a small litterbox (boxes made for ferrets work well for kittens) and put about an inch of litter in it. Don't use clumping litter, as it can get stuck in kitten paw pads. Put a box in each room the kittens are allowed in, so they don't have to go far to "go".
Pick them up, one by one, and put them in the box. They'll probably jump right out, but they'll know where it is then. Next time they look like they are going to squat or look like they need to use it, put them in the box. Sometimes it helps to move their paws to dig.
They'll get the idea easily, since it's instinct. If/when they have accidents, rub their noses in the pee and then immediately put them in the box.