
I normally start my medium size breed with litters of 4+ at 18 days.
I generally find that the smaller litters are fairly uninterested at first, and really only tasting, but it ensures that should they need it they have food other than Mum's milk, and large litters often quite keen.
I currently have a litter of three and I could barely get them to even taste soaked puppy food until nearly 4 weeks, though offered it from 3 weeks.
In fact they barely ate more than a handful between them per meal until a week ago, and they are 7 weeks old, but Mum still feeds them several times a day. Her milk must now be diminishing as they are now finally eating what I would expect for this age.
I start with one taste, then two the next day, then three and then four times a day increasing amounts to appetite.
Mum will happily finish off what they mess about and leave.
As to what you feed there are varied regimes. My own was to soak dry pupy food, mash it and serve with some goats milk based puppy milk. Others will start with scraped beef, others tinned puppy food and others raw natural feeding.
My own preference is a good quality complete, (with the occasional novel item, like scrambled eggs, chicken wings or thighs) as unless new owners are experienced raw feeders I prefer that puppies with their rapid growth have a balanced diet, as there is less room for error doing harm in fast growing pups.