A daily ferry travels regularly in the morning from Ko Lanta to Krabi and back, stopping halfway at large of Koh Jum, where you are taken by longtailboat. Depending on season and demand, it may make a second journey in the afternoon. Travel time about one hour from Krabi or Koh Lanta, october to april.
Coming from Krabi town or Krabi airport you can go by taxi or van to the village of Laem Kruat (about 20 km). There leave regular longtailboats to Koh Jum Village or Koh Jum Pier, whence you have another 4 km across the island to get to the resort.
Or else have your longtailboat directly from Laem Kruat to the resort - it takes half an hour and is a great and very feasible arrival, please inquire with the resort.
You may arrive also from the Phi Phi islands by longtail boat, travel time about one hour.