Circe tells Odysseus what to do to gain access to the Underworld. She says, in part, to
"[...] sit back and the North Wind will speed you on your way [...]. Beach your vessel hard by the Ocean’s churning shore and make your own way down to the moldering House of Death. And there into Acheron, the Flood of Grief, two rivers flow, the torrent River of Fire, the wailing River of Tears that branches off from Styx, the Stream of Hate, and a stark crag looms where the two rivers thunder down and meet."
This is how they will get there, then. His ship will take him to the proper place, and, there, Odysseus must secure it and go, on foot, into the Underworld. He must cross the rivers that border it and run through Hades until he reaches the place where they meet. Once there, he must go bravely forward and dig a trench about "a forearm's depth and length," and pour out "libations" as offerings to the dead. First, he must pour "milk and honey," then "mellow wine," and, finally, "sprinkle glistening barley over it all." He also has to promise to slaughter animals to offer the dead when he gets home to Ithaca.
Finally, Circe says,
"And once your prayers have invoked the nations of the dead in their dim glory, slaughter a ram and black ewe, turning both their heads toward Erebus, but turn your head away, looking toward the Ocean River."
In other words, on top of the milk and honey, wine, and barley, Odysseus must pour out the blood of both a ram and a ewe so that the shades can come forward and drink. He must then have his men burn both carcasses for Persephone and keep the other shades at bay until Odysseus has "questioned Tiresias" himself.