Separate the buck ASAP! He will definitly impregnate the doe right again. Most likely he already did if you didn't watch her giving birth and every moment after. That's one of the less cute behaviours of rabbits, there's little that gets a buck more horny than a doe giving birth.
Then check the nest, replace soiled, wet bedding, and make sure the kits are well covered with fur. If there isn't enough fur you can supplement with cotton wool or dryer lint if you don't have any fur from brushing rabbits.
Other than that, give the doe privacy, rest, lots of food, nesting material if she feels the need to use it. Just avoid any stress.
It can take 24h (even longer in rare cases) for the milk to drop, so be sure the kits are insulated well and leave the doe and kits alone. Some does do not nurse as long as anyone is around.
