I've ridden through Ban Hat Lek (KKong) many times. Below is some (overly) detailed info:
Take passport, bike's green book and some passport photos. Also take a copy of the Thai
insurance doc for your bike as a precaution (often not checked).
Upon approaching Trat, the 90km road to Ban Hat Lek (Thai border) is just before entering the main part of the city. You may want to enter Trat and get something to eat first, but there are places enroute to the border. At Klong Yai (about 10-15km?) before the border, I recommend filling your gas tanks; gas is more expensive in Cambodia.
Procedure at the Thai border (Ban Hat Lek):
1-Go to the south end (it faces Cambodia) of the Immigration bldg with you passport and bike's green book. I assume the bike is regged in you name otherwise you need to have a certified doc from the bikes owner which explicitly give you permission to take the bike to Cambo (the latter isn't alway accepted by Immigration!). At the window they will fill out a temporary import/export doc for the bike. Keep these docs.
2-Go the the side of the Immigration bldg, with green book, im/ex docs and passport, and check
out of Thailand.
3-Go across the street to the Customs bldg. Give them passport, green book and the im/ex docs. They'll enter the info in their computer and give you back 2-copies of the im/ex doc. DO NOT lose them as they are needed when you re-enter Thailand
4-Your finished...now ride the 200-yards to Cambo Immigration; the white bldg on the left.
Procedure Cambo Immigration (Cham Yeam):
Note- In Cambo you drive on the right-hand side of the road...not left.
I highly recommend obtaining your Cambo visa either from their BKK Embassy or by e-mail
[e-mail: here ... ]. Actual visa cost is $20-US and $25-US for the e-mail visa (30-day visa). Pay for visa with $US if obtained at the border. However, at the border will will surely be overcharged for the visa, many have been gouged for as much as 1500-Baht!
1-Window on left...get form and fill it out, then enter door to right, give them form, passport and one photo (and copy of e-visa if obtained). Once you have visa and passport back, you're good to go. You do _not_ need to check/register your bikes with Cambo Immigration, but often
some people outside will ask you to reg the bike...ignore them.
From Cambo Immigration it's 11km to Koh Kong. You'll need to stop at another Cambo checkpoint (shack) about 4-km from Immigration. They will check that you have a visa then oneward to KKong. Pay 12-baht at the toll booth, cross the bridge and at the round-about just continue straight and you'll encounter a good road (Hwy-48) that goes the 138km through the Cardamon mountains, crossing 4-bridges enroute. Shortly after the last bridge (Sre Ambel) you'll hit a 'T'...go left to PPenh, right to Snooky. I suggest considering stopping in KKong, go the market to one of the money-changers and change a few $dollars into Cambo 'Riel'.
Be careful riding in Cambo...drivers are erratic; oncoming cars/buses don't hesitate to use your lane to pass.
Have fun.....