Andaman Dive Adventure Lanta

From Bangkok

You can take either an airplane (Air Asia to Krabi or Air Asia or Thai Airways to Phuket or Nok Airto Trang; takes roughly 1 hour), bus to Krabi (most comfortable is the VIP bus; takes roughly 11 hours, leaves around 5 PM from Bangkok) or train to Surat Tani (after this you still need to book a minibus to Krabi; whole trip takes about 15 hours, involves a lot of waiting, night train leaves around 4 PM). Of course the first option is more expensive, but saves you a lot of time. From Krabi it takes roughly 2,5 hours, from Phuket roughly 5 hours and from Trang roughly 2,5 hours to get to Koh Lanta by minibus. Please double check the times posted for bus and train & pre-reserve these in high season or during holidays.

From Krabi or Trang

Air Asia allows you to book all transportation until Koh Lanta (in high season they use minibus & ferry; in rainy season just minibus). Otherwise you have two options: minibus (takes around 3 hours, cost around 400 Thai Baht) or private taxi (takes around 2,5 hours, cost around 2500 Thai Baht per taxi or minivan) to Koh Lanta.

From Phuket

Air Asia allows you to book all transportation until Koh Lanta (in high season they use minibus & ferry; in rainy season just minibus). Otherwise you have two options: minibus (takes around 6 hours, cost around 800 Thai Baht per person) or private taxi (takes around 4,5 hours, cost around 4500 Thai Baht per taxi or minivan)

From Kuala Lumpur

You can get to Koh Lanta by airplane (Air Asia to Krabi or Air Asia or Malaysia Airlines to Phuket),bus or train (see above comment about trips from Bangkok, around the same departure times, takes a long time to get to Krabi and Koh Lanta; the train goes to Hat Yai, then you are still a 5 hour drive by minibus from Koh Lanta).