How is the death of Candy's dog foreshadowing?

Foreshadowing in literature is an event that happens early in a book or story that gives a clue or idea about what is to come later on.
Candy shooting his dog foreshadows George shooting Lennie at the end of the book.
Most of the ranch hands in the book are lonely because they wander from job to job. They can't put down roots, and they can't afford to marry. Candy has a dog for companionship, but the dog is getting old and infirm. It smells bad, and some of the other ranch hands who don't want it around pressure Candy to shoot it and put it out of its misery. Candy loves the dog and doesn't want to kill it, but does not feel he has an alternative.
Likewise, George loves Lennie and enjoys his companionship, but at the end of the book feels he has no choice but to shoot him.

The death of Candy's dog in chapter 3 is foreshadowing because Steinbeck has drawn parallels between the relationship of George and Lennie and of Candy and his dog. After allowing Carlson to shoot his old dog, Candy tells George, "I oughtta of shot that dog myself, George. I shouldn't oughta of let no stranger shoot my dog." This puts the thought in George's head that when the time comes, he should be the one to take out Lennie rather than allowing any stranger to kill Lennie.

At the conclusion of the novel when George shoots Lennie to spare him an agonizing death at Curley’s hands, the reader is reminded that a variation of this tragedy occurs earlier in the narrative when Carlson shoots Candy’s old dog to end its misery. Carlson’s shooting the dog humanely in the back of the head foreshadows the manner of Lennie’s death, and like Candy, who consents to putting his dog down, George experiences heartbreaking anguish in doing what must be done. In the context of the novel, Candy’s bond with the old dog is as meaningful as George and Lennie’s friendship, as both relationships forestall loneliness and give purpose to the men’s lives. Candy’s despair as he lies in his bunk and turns his face to the wall after his dog is shot foreshadows George’s feelings of profound loss as he sits on the riverbank next to Lennie’s body.
The parallels between Candy’s and George’s behavior as they eventually accept what they must do emphasize that the scene in the bunkhouse foreshadows the concluding scene at the river. Candy resists agreeing to shoot his dog, delaying the inevitable as long as possible. “Maybe tomorra. Let’s wait till tomorra,” he tells Carlson. Sitting next to Lennie by the river, George also delays the inevitable. Hearing the search party closing in, he says good-bye to Lennie in his own way until time runs out. With the search party too close to ignore, George shoots Lennie humanely in the back of the head with the Luger Carlson used to shoot Candy’s dog.