At first, this will be when you weigh them after birth. In addition to putting them on the scale, you should hold the puppy, stroke it, and talk to it. They cannot hear you, but they can feel the vibrations from your speech. Individual handling should increase in amount and duration as the puppy grows. Walk about the house with it.
Different rooms have different smells, temperatures, and sounds.
This should continue when the puppies' eyes and ears are open. Individually, each can be put on the floor and allowed to walk about and explore outside the litter box and without the security of mom and siblings. You will have to monitor the puppy's reactions. He can easily get into trouble. He can be mildly fearful but should be able to allay those fears in a short time.