I start my litters on puppy food at around 3 1/2 weeks of age. Bitch is still nursing as she wants to.
I start slow to avoid loose stools. I feed Orijen so I start with 1/4 cup per puppy.
This is for a litter of 8: Soak 2 cups kibble in water till soft. Put it in the blender and add 1/4 cup baby rice cereal (helps avoid loose stools). Blend well to make a semi thick mush adding tepid water as needed.
Feed pups and see how much they eat. Do this once a day for a couple of days and up to 2 feedings a day. Slowly increase the amount per pup by going up 1/4 cup intervals. And get them up to 3 meals a day.
The amount you feed will be determined by how they eat. I like to have a bit left in the bowl so I know they are getting enough.
At around 6 weeks of age I stop blending and just mash a bit with a potato masher. Adding rice or not is determined by firmness of stools.
Eventually I feed just soaked kibble and serve. Once I know there is no choking during eating I feed kibble with tepid water added (no soaking)
By the time my pups go to their forever homes at 8 1/2 weeks they are eating about 1 cup 3 times per day.