When Odysseus is preparing to leave Circe's island for the first time, she tells him that he must travel to the Underworld in order to seek the guidance of the blind prophet, Teiresias of Thebes, who -- though dead -- still has his mind because Persephone has granted him reason and understanding. He does so and Teiresias tells him what his way home will be like.
After Odysseus returns from the Underworld, he goes back to Circe's home in order to retrieve the body of Elpenor, a young crew member who died just as his men were preparing to leave before. Circe receives him, and he and his men spend the night. She tells Odysseus that he will have to go past the sirens, and she tells him how to handle them. Further, she explains that his ship will have to pass either Scylla or Charybdis, and she advises that they sail closer to Scylla (because the monster can only kill six of his men at once whereas Charybdis could, theoretically, sink the ship and kill them all). Finally, she says that they will have to pass Thrinacia and must be sure not to eat any of the Sun God's sheep there.