Specifically, how does foreshadowing help prepare the reader for what happens to Curley's wife and Lennie's death?
Steinbeck prepares the audience for the deaths of Curley's wife and Lennie early on in Of Mice and Men. In the novel's first chapter, George reminds Lennie of the "bad things" that he did in Weed. George also reminds Lennie of how Lennie had just wanted to touch a certain woman's dress (because the dress was very soft), an action that the woman regarded as an unwanted sexual advance (see Chapter 3). Lennie's attempt to touch the woman in Weed anticipates his touching of the hair of Curley's wife later, a touch which results in him accidentally breaking her neck.
Also, in Chapter 1, Lennie's petting the dead, soft mouse also foreshadows his petting of the soft puppy, which he kills just before he kills Curley's wife.
The killing of Candy's old dog in Chapter 3 foreshadows Lennie's own death in the novel's final chapter. Unlike Candy, however, who allows Carlson to shoot his dog with the Luger, George will be the one who kills his constant companion Lennie:
He reached in his side pocket and brought out Carlson’s Luger; he snapped
off the safety, and the hand and gun lay on the ground behind Lennie’s back.
He looked at the back of Lennie’s head, at the place where the spine and skull
Thus, by the time readers come to Chapter 6, Steinbeck has well-prepared his audience for the deaths of both Lennie and Curley's wife.