Socialisation is the single most important thing you will do with your puppy, it happens in a window that closes, and almost everybody misunderstands what it means.
It does not mean meeting as many dogs as possible. A puppy dragged around dog parks meeting forty strange dogs is not being socialised, it is being overwhelmed, and you can produce a reactive adult that way.
Socialisation means controlled, positive exposure to the ordinary world, at a level the puppy can handle, before roughly sixteen weeks. The goal is a dog for which nothing is news.
The window
Roughly three to sixteen weeks. We do the first half here: household noise, the vacuum, the doorbell, visitors, different surfaces, being handled all over, being briefly alone. You get weeks eight to sixteen, which is the part that decides how your dog handles your world.
After sixteen weeks a puppy’s default shifts from curious to cautious. You can still do good work after that, and plenty of people rehabilitate under-socialised dogs, but it is remedial rather than foundational. The window does not reopen.
The rule that makes it work
Watch the puppy, not the checklist. Every exposure should end with a puppy that is relaxed, curious and interested. If it is frozen, tucked, panting, or trying to leave, you have gone too far, and repeating it will make it worse rather than better.
Back off to a distance where the puppy is comfortable, let it watch, feed it something excellent while it watches, and let it approach in its own time. Distance is the dial you turn. That is the whole technique.
What to work through
People
Men with deep voices. Beards. Hats. Hoods. Sunglasses. Umbrellas. High-visibility jackets. Walking sticks, crutches, wheelchairs. People running past. Children, at a distance first, and calm ones before excited ones. Someone in a delivery uniform who gives the puppy a treat is worth ten trips to the park.
Sounds
Traffic, sirens, motorbikes, the vacuum, the washing machine, the hairdryer, fireworks and thunder recordings played quietly and built up slowly, dogs barking, doors, the letterbox.
Surfaces and places
Grass, gravel, tarmac, tile, metal grates, wet ground, wobbly things, stairs. The car, on short and pleasant journeys that do not always end at the vet. The pavement outside a supermarket, sitting and watching the world go past.
Handling
Feet, between the toes, ears, mouth, tail, belly. Nail clippers touched to a nail and then a treat, long before you actually cut anything. A brush over the coat. This is a fortnight of easy work that buys you twelve years of straightforward vet visits and grooming.
Other dogs
Quality over quantity, always. A handful of known, healthy, fully vaccinated, well-mannered adult dogs will teach your puppy far more than a park full of strangers. A sensible adult dog telling a rude puppy off, appropriately, is a genuinely valuable lesson.
Doing it before the vaccinations are finished
This is the tension everybody runs into, and the answer is not to wait. Carry the puppy. Drive it places and let it watch from your lap. Invite people to your house. Use your own garden. Visit friends with healthy vaccinated dogs. Avoid public ground where unknown dogs relieve themselves until the course is complete. The vaccination page covers the timing.
Being alone
The exposure nobody remembers. Start on day two: sixty seconds in a pen with something good to chew, while you are visibly nearby, then build. A puppy that never learns to be alone becomes a dog that cannot be left, and that is a hard problem to fix later.
A reasonable pace
Two or three new things a day, each of them short and each of them ending well. Twenty minutes of good exposure beats an exhausting afternoon. Puppies consolidate what they have learned while asleep, and they sleep a great deal, which is not laziness but the work happening.
The socialisation checklists we link on the resources page are worth printing and putting on the fridge.