The foreshadowing is created by building momentum. In a slow, steady way, Shirley Jackson tells us about the daily dalliances of the villagers on the day of the lottery. The author does not offer any information about the lottery itself, though. We learn from it through inferences until the very moment when the event begins at the end of the story.
In the first paragraph, the only thing we know that hints at something about to take place is that the people began to
gather in the square, between the post office and the bank, around ten o'clock
This hints at internal knowledge that the townspeople have and share, but is unknown to the reader. We know something must be going on, but we aren't quite sure yet.
The event of the lottery is the focus of the paragraph, and it is further explained by stating how, in this village, the lottery is different than everywhere else. In this village, the lottery only took two hours so people could get home in time for lunch. Here we come to realize, as readers, that this lottery is either mandatory or so ingrained in the minds of the villagers that they must conduct it no matter what. Hence, the lottery must be a big deal.
In the second paragraph, Jackson gives us another piece of information: The kids were out of school for the summer and so they gathered first, some with a pocket full of smooth, round stones.
Bobby and Harry Jones and Dickie Delacroix—thevillagers pronounced this name "Dellacroy"—eventually made a great pile of stones in one corner of the square and guarded it against the raids of the other boys.
The author uses foreshadowing a couple of times in the first three paragraphs to show how a seemingly pristine summer day with laughing and playing children is not necessarily what it seems.
The story begins on a "clear and sunny" day with flowers "blossoming profusely and the grass was richly green." The children, who are still thinking of school, "broke into boisterous play."
Then the narrator begins to foreshadow what the lottery really is by sandwiching seemingly throwaway lines in between the discussion of play and girls "looking over their shoulders at the boys." The narrator describes how Bobby Martin "had already stuffed his pockets full of stones" and other boys were doing the same. The narrator also explains the pile of stones the boys "guarded" against the raids of other boys. These seemingly innocent statements of boys playing with rocks foreshadow the ending of the story.
In addition, as a hint as to the violence that was to occur, the older people in the town gathered and told jokes that "were quiet" and the people "smiled rather than laughed."
These events clearly foreshadow the events that occur at the end of the novel, but, on an initial reading could be passed over as incidental characterization of the members of the village.