You fly. That is the short answer, and it is nearly the only answer.
Molokaʻi has no direct service from the mainland, so every trip routes through Honolulu or Kahului. From Honolulu the flight to Hoʻolehua, the island's airport, takes about 25 minutes.
The ferry stopped running
This is the thing most people are actually searching for, and the answer is no. The Molokaʻi Ferry between Lahaina and Kaunakakai ended service in 2016, and nothing has replaced it.
Old travel articles and forum posts still describe it, which is why the question keeps getting asked. If a page tells you to take the ferry from Maui, it is out of date.
What the flight is actually like
The aircraft is small. Expect a nine seat turboprop, no cabin crew, no jet bridge, and a walk across the tarmac. For most people it is the best part of the trip: you fly low, the whole flight is a window seat, and on a clear day the Molokaʻi sea cliffs from the air are worth the fare on their own.
Two practical consequences of a small aircraft. Weight matters, so passengers and bags are weighed and there are real limits. And weather cancels flights more readily than it does a jet, so do not book a tight same-day connection back to the mainland. Give yourself a night on Oʻahu at the end.
Book the car before the flight
This is the mistake worth avoiding. There are very few rental cars on Molokaʻi, and unlike Maui, where running out means paying too much, running out here means there is nothing available at any price.
The island is around 38 miles end to end with no public transport worth planning around, so arriving without a car is a genuine problem rather than an inconvenience. Book it first, then book the flight to match.
Check Molokaʻi rental availability
Where you will land
Hoʻolehua Airport sits in the middle of the island, about eight miles from Kaunakakai, the main town. It is one room. There is no jet bridge, no baggage carousel worth the name and no line.
Kalaupapa has its own separate airstrip on the peninsula below the cliffs, and it is not a way into the rest of the island. Access to Kalaupapa is restricted and permit-based, and conditions change, so check the National Park Service page rather than anything written in advance, including this.
Before you book anything
Lodging on Molokaʻi is limited to a small number of places, and it is not a matter of picking a favorite from twenty. Sort accommodation at the same time as the flight and the car, not after.
Whether the trip is a good idea at all is a fair question, and covered here honestly.
