Good socialisation is not meeting as many dogs and people as possible. It is a series of calm, positive experiences where your puppy is never frightened and always able to retreat. For a sensitive breed like the Bernese, that distinction is everything.
Start with what you can control at home: floor surfaces, household sounds, being handled, a crate, a car with the engine running. Because our puppies meet all of this before they leave, you are continuing a process rather than starting one.
Then widen carefully. One new thing per outing, watched from a comfortable distance, ending before your puppy tires. A puppy who observes a busy street calmly from a bench has learned more than one dragged through the middle of it.
Watch for the quiet signs of too much: yawning, turning away, sudden stillness, refusing food. Give space immediately and try again another day, smaller. Berners rarely need repetition; they need not to be pushed.