I am not a breeder but you definitely want to introduce food slowly and gradually and make sure it is very mushy. It's going to be messy! Don't worry if they don't eat it right away, they may just play around in it a few times. Go with a high quality puppy food (look at dogfoodanalysis.com for some examples). I would start around 5-6 weeks so by 8-9 weeks they are hopefully mostly weaned off mom and make the food less mushy each week.