Getting to us from Bangkok
Depending on your budget and available time, there is a few ways of getting to us from Bangkok.

You can either take a plane from Bangkok to Phnom Penh or the most scenic and budget friendly way is to get here by bus.

The most exiting way is to do it yourself:
Here is how to #1
From Bangkok Ekamai Bus Terminal there are at least two bus companies with departures almost every hour going to Trat, Which is on the way to Koh Kong. (NB: Koh Kong is the name of the border city between Thailand and Cambodia, but also an Island and a province).
You get to the Bus terminal either by taxi or use the SkyTrain, Sukhumvit line to Ekamai station.
The price is from about 240thb per person and the trip to Trat takes about 6 hours +/-.

From Trat bus terminal you take the bus to Koh Kong.
There are departures to Koh Kong almost every hour.
Price is from 140Thb pr person and the trip to Koh Kong takes about 3 hours +/-
If it is getting close to dark, we suggest to spend the night in Trat (Try out Artist Place).
How to get to us from Bangkok, Kao San area by bus
How to #2
Go to a local ticket office in Kao San and buy a ticket to Koh Kong
about $20 pr person and the trip takes a full day, with some changes from tuktuk/moto, to big bus, to small bus, to tuktuk and back to small buses again.
Trust the system and bring some snacks and drinks.

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.

Get a private car from the border to us (if anyone of the drivers know where Koh sdach is) $60 is a good price and the trip takes about 3 hours.
Or 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 shared cars only leave once a day around noon.