Why the window matters
Between about 3 and 14 weeks, puppies are primed to accept new things as normal. After the window closes, unfamiliar things are increasingly treated with suspicion. You cannot socialize a puppy too much in this window, but you can socialize badly: flooding a puppy with scary experiences does more harm than good. The goal is positive exposure, not just exposure.
Note on vaccines: talk to your vet about the balance of disease risk and socialization in your area. Most vets now support careful socialization before vaccines are complete: carry the puppy in busy places, invite healthy vaccinated dogs to your home, and avoid high-traffic dog areas until cleared.
The checklist
People: men with beards, people with hats and sunglasses, kids of different ages, people using canes or wheelchairs, delivery workers, people in uniforms. Sounds: vacuum, hair dryer, doorbell, thunder recordings (start quiet), traffic, sirens at a distance, applause. Surfaces: grass, gravel, wet pavement, metal grates, slippery floors, sand, stairs. Places: pet-friendly stores, outdoor cafes, the vet clinic just for treats, friends' houses, near playgrounds (at a distance). Handling: ears, paws, mouth, being picked up, wearing a collar and harness, baths. Other animals: calm vaccinated dogs of various sizes, cats at a distance if relevant.
How to socialize right
Pair every new thing with treats and keep sessions short. Let the puppy choose to approach; never drag them toward something scary. Watch body language: loose wiggly body means keep going, tucked tail or freezing means add distance and try easier. One positive five-minute outing beats an hour-long overwhelming one.
Keep a list on your fridge and check things off. It is satisfying, and it makes sure the quiet weeks do not slip by with nothing new.
What to avoid
Dog parks with unknown adult dogs are a gamble for young puppies: one bad experience can undo weeks of good work. Forced greetings with every stranger teach the puppy that people always grab them. And do not comfort a scared puppy by tightening the leash and hovering; calmly add distance and let them recover, then reward bravery.