Socialization is the small window of neurodevelopment when a puppy or a kitten are most responsive to learning, and this is the best time to teach them the life skills they will need to thrive.
At this time, an owner should give their kitten/puppy as much POSITIVE exposure to anything they will need to be familiar with, like other animals, people/children, strange noises, new environments etc.
As an owner, make sure you know the signs of fear, no matter how subtle, your new friend shows, and allow them to approach the new factor at their own pace and allow them to retreat when they become anxious or overwhelmed.
Encouragement to form positive associations can be given in the form of treats/toys.
Your veterinary clinic should be apart of your new friends social experiences. It is recommended that you bring your puppy/kitten in to the vet for purely social visits (that do not include vaccinations), so they have positive experiences with being handled by veterinary staff. It is also recommended that your friend is given small, frequent treats while getting vaccinations so they form a positive bond with your veterinarian.
Puppy preschool/kitten kindergarten socialization classes are recommended, as they can improve owner education and positively exposes your puppy/kitten to new and various stimuli in a nonthreatening way for positive development of life skills.
(All information obtained from Katherine Pankratz, DVM, DACVB as presented at the 2020 Applied Animal Behavior conference hosted by the UW Madison School of Veterinary Medicine)