how does odysseus gain access to the underworld?

Circe instructs Odysseus to do the following. He must travel to the land of the Cimmerians. He must dig a trench once he arrives and pour into it a drink-offering to all the dead, first with honey and milk, then with wine, and finally with water, and then he should sprinkle with white barley meal. After proper prayers, next he must sacrifice two sheep (one black ram just for Teiresias) and let the blood flow into the trench before burning the animals. The shades (ghosts) in the Underworld will be attracted to the pit to drink the blood, but Odysseus must hold them off to allow Teiresias to drink first. From Teiresias, the blind prophet, Odysseus will learn what he must do to return to Ithaca and to make peace with Poseidon.