Many travellers use the Hat Lek–Koh Kong border crossing for overland travel into Cambodia. It’s best to arm yourself in advance with an e-visa for Cambodia and to make the journey by regular public transport, but it’s also possible to buy a package all the way through to Sihanoukville and Phnom Penh and to get a thirty-day visa on arrival at the border, though both of the latter options are more likely to open you up to possible scams.
The only way to get to Hat Lek under your own steam is by minibus from Trat bus station, 91km northwest. Hat Lek (on the Thai side) and Koh Kong (in Cambodia) are on opposite sides of the Dong Tong River estuary, but a bridge connects the two banks. Once through immigration, taxis ferry you into Koh Kong town for onward transport to Sihanoukville and Phnom Penh or for guesthouses should you arrive too late for connections (mid-afternoon onwards). Vans, buses and share-taxis to Phnom Penh and Sihanoukville take around 4–5hr.