How did Odysseus end up back in Ithaca sleeping on the beach?

Odysseus reaches Ithaca thanks to the Phaeacians, who listen to his incredible story and decide to help him get back to Ithaca after twenty years of traveling. King Alcinous and Princess Nausicaa give Odysseus a ship and many gifts to take home, and Odysseus sails back to Ithaca safely. He disguises himself as a beggar before proceeding towards the town. He is not shipwrecked and does not sleep on the beach.
The island where Odysseus is shipwrecked and is found in a deep sleep on the beach is not Ithaca but the island of the Phaeacians. Before reaching the island, Odysseus was supposed to be sailing home. Earlier in the story, in fact, Odysseus leaves Calypso with his men in order to return home. However, the god Poseidon sends out a storm which causes Odysseus's ship to sink. In fact, Poseidon is angry because Odysseus previously blinded his son Polyphemus (a cyclops who, in Odysseus's defense, was actually trying to eat him and all his men). Odysseus ends up as the only survivor on the beaches of Scheria, the island of the Phaecians, where Nausicaa finds him sleeping on the beach.
Nausicaa then takes him to the palace, where the Phaeacians listen to his story and decide to help him. Hopefully this clears up any doubts about the two different islands!