From Krabi pier:

Buy your ticket (approx. 100 baht) to the island "Koh Pu" (also known as Koh Jam, the largest village on Koh Pu). Make sure to tell the ticketoffice/salesmen on the pier that you are NOT going to Koh Jam, but the small "village" on the north/western end of the island.

The ferry is the same ferry that goes to the large island "Koh Lanta", but will stop halfway to Koh Lanta at Koh Pu.

After approx. 1 hour, the ferry will stop in open sea, where a small fisherboat will be waiting to take you to the shore. If you have not made your reservation, make sure there are free bungalows on the resort before leaving the ferry.

When you want to leave, the small fisherboat will take you out to the ferry, where you will buy your ticket onboard.