From Koh Chang and Trat, Cambodia is only a bus ride away.
There are departures to Koh Kong almost every hour from Trat bus terminal.
Price is from 140 THB pr. person and the trip to Koh Kong takes about 3 hours +/-.
If you have arrived from Koh Chang to Trat and it is getting close to dark we suggest to spend the night, (try out Artist Place).
On the Cambodian side of the Thai/Cambodian border there are many people trying to help you with passports, visa, taxis etc. and they all want a piece of your money.
Haggle the best way you can, always with a smile, that helps.
On the border you can get a private car to us (if any of the drivers know where Koh Sdach is) $60 is a good price and the trip takes about 3 hours.
Or you can get a tuktuk/shared car for a few dollars to Mr. Ritthy in Koh Kong and he can help you with a shared car to Poi Yopon for $10 pr person. Note the local shared cars only leave Koh Kong once a day, around noon.