Thailand's legendry bus service has direct routes to Ranong from Phuket, Surat Thani and Bangkok. From other destinations you will most likely pass through one of these and change vehicle.
Standard government fares: Bangkok - 350Th.B, Phuket - 200Th.B, Surat Thani - 220Th.B

When you arrive at the main bus terminal, taxis will already be waiting and just ask to go to the Koh Chang/Koh Phayam pier. Koh Chang is only about an hour by longtail boat, Koh Phayam an extra 20mins.

Boats leave daily and during high season (Dec - Apr) normally leave once at midday, once in the late afternoon. The information booth at the bus station will have up-to-date information. The fare is approx. 100Th.B. If you already know where you'll be staying, some resorts will send their own boat to meet you at set time.

For unknown reasons, there is no recognized service between the two islands. It usually requires hiring a private boat from one of the resorts which can be expensive. The more people, obviously the cheaper the trip. Alternatively, head back to the pier in Ranong and get the scheduled boat. A pain-in-the-arse for sure!

Bus Description Ranong - Bangkok
Co.Ltd.	air 2.	air 1.	VIP 32	VIP 24	Phone.Bangkok	Phone.Ranong
Transport	350 Th.B	428 Th.B.	non	665 Th.B	435-1199, 434-5557-8	(077) 811548
Chokanan Tour	non	445 Th.B.	517 Th.B	680 Th.B	435-7429, 433-0723	(077) 811337
Mit Tour	non	445 Th.B.	517 Th.B	680	281-6939, 281-6949, 435-5020	(077) 811150