Socialising is all about gradually increasing without over faceing the pup. Not adding to the stress levels until you are sure that pup can cope.
What a dog has to cope with depends so much on where the owner lives and what their interests are. For instance, a pup who's future is mapped out as a working gundog needs to be bomb proof around livestock, where a dog living in the middle of London may never see a field of sheep!
So give some thought to what your pup needs to know about and rank from the least frightening upwards. Then set out to visit each of these things in turn. But be aware, a pup may not view things in the same way as us. I remember my old Katy's big fright was a piece of white polythene flapping in a bush! I quietly picked it up and walked on carrying it so she had plenty of time to see it, and more importantly, see that it was not hurting me. She was never bothered about flapping things again!