Write a list of all of the plot points you wish to portend.
Determine what class of omen you want to use for your readers. To do this, you need to know what kind of story you want to tell. For example, a story that takes place in nature (with bugs as characters, or a story about trees) might use natural omens. A story about Greek gods might use traditional Greek omens, such as birds or dreams. There are several classes of omens, including chance meetings and astrological/astronomical omens. There are also physiognomic omens (the appearance of a character predicts a certain outcome) and weather omens.
Connect an omen to each plot point you wish to augur or foretell. For example, if you chose physiognomy as your class of omen and want to portend a trip to Saturn, you could make the main character's eyes dusty brown at the edges (like the dust rings of the planet) with icy blue spots (representing the icy moons).
Determine where to place each plot point in the story. Omens that portend the rising action should appear near the beginning of a story, while omens that predict a resolution might need to appear closer to the climax.