This is what we do....
Collar the pups as they are born. Document everything about them. We use those colored hairbands women use for ponytails. Not tight, though. They come in a variety of sizes.
Get a fish scale or a reliable scale that you van measure up to 8 pounds. Weigh the pups every other day.
You will see steady growth. Any pups starting to stay the same weight or lose weight get one on one mom time.
At some point, in about three and a half weeks you will notice a leveling offon all the pups weight. That is the time to introduce soft food.
All that is personal preference.
We put about a teaspoon of food on a plate at first and let the pup lick it. Next day, add a little more. We also start each one individually so there is no competition at first. They still nurse for a few more days, but mom will start to reject them. At that point we let mom do what she wants.
Anything else is breed specific, so talk to other peopme who breed that type of dog for more info.