You don't have to do anything. Just keep providing Mama with high quality canned and dry kitten food. As the kittens approach their 4 week birthdays, you will need to start putting the food in shallow dishes if you are not already doing so. The kittens will begin to nibble canned and dry food somewhere between 3.5 and 6 weeks (usually between 4 and 5 weeks). The kittens will begin solids on their own time and Mama will handle the weaning process in her own way. All you need to do is make sure the food is available.
Also, you will need to provide the kittens with a litterbox beginning around their 4 week birthday. Use any type of litter except regular clumping (the clumping litters made from corn or wheat are fine and actually ideal for these purposes if you can afford them). The kittens can use Mom's litterbox if the sides are low enough, but I personally prefer to use foil roasting pans for little kittens' litterboxes. The sides are only a couple of inches high so the kittens can get in and out easily.