Establish the key plot points in your story. What happens? How does the story end?
Look over the entire story and search for ways in which you can allude to important plot points or to the ending. For instance, if your story is a murder mystery and you know who committed the deed, then return to the point in your story when the murderer is first introduced. If your story is about the break up of a marriage, turn to the first time the married couple is introduced and look at how the husband and wife interacts with one another. These can be a key areas to foreshadow the story’s ending.
Write something about the character that will alert to your readers about his importance to the story. For instance, if he is the murderer in your crime novel, use character description or a back story as a way to foreshadow his importance to the plot. Don’t be obvious. You don’t want to give yourself away too easily.
Look at the dialogue. This can be used as a way to foreshadow your story as well. Again, don’t be obvious. Plant little clues in the character’s dialogue that will allude to the story’s ending.
Use metaphors or similes. For instance, in the murder mystery, the murderer can be compared to something that evokes death or destruction in your readers’ minds.
Search for objects that you think will allude to important plot points of the story. Determine how you can use that object to foreshadow the ending. For instance, a pair of gloves found at the scene of the crime can be used to solve the murder. Posters of famous rock stars who died at a young age plastering a young teen's bedroom walls can foreshadow a sense of doom for the character.
Use symbolism. For instance, an ominous setting or weather pattern (“the gathering storm clouds”) can foreshadow what will happen in your story. Animals, such as crows or vultures, are also obvious symbols that can be used to foreshadow an important plot point or the ending of your story.