Arriving at the border. It's a walk of 100 to 500 meters, depending on where you park or where the bus drops you. You may opt for a local form of taxi to take you all the way.
First and foremost: You do NOT need help from any of the many hustlers lingering at and around the border. They will offer to guide you, to provide forms, to get you a visa. You do NOT need any such services. "Mai ao, krup" ("I don't want it") works if you repeat it over and over. You may add an emphatic "Pai," ("Go!") if they become real jerks.
As you approach the border crossing, walk to your left and enter the first door of the first building on your left. Once inside, use the right line ("foreign passport holders") to process out of Thailand. That part is fast, easy and free. Exit that first building, turning left over the border into Cambodia. As you do so, look for the Thai Immigration booth across the road (to your right). Stop there only long enough to pick up a TM form (fill it out later), then continue on into Cambodia, stopping at the first building on your right–the Cambodia Immigration booth.
On the counter, you will find applications for entry. Fill out the simple form (instructions and sample form are displayed in window), then turn it in at the window along with your passport, a passport photo, and 1,000 baht.
Now, while waiting 10 to 20 minutes for the return of your processed passport, fill in the Thai TM form. Keep your eye on the processing window as the Immigration officers will NOT call your name; they simply place a pile of passports on the counter. (You don't want someone else walking off with your passport.)
Continue further into Cambodia (only another 30 meters or so) where there are several long tables in front of a structure to your right. On the first table you will find Cambodian Visa forms. Complete one form and take it to the Immigration officers just to your right. There are two lines, one marked "Arrivals," and one marked "Departures." You will need to wait in both, beginning with the line on the left ("Arrivals"). There you will give the officer your passport, the visa form you just completed, and 100 baht. He will return your passport and visa form which you then immediately take to the adjacent line ("Departures"). There you will be processed out of Cambodia.
Finally, return to the Thai Immigration office (where you picked up your TM form) and give them your passport and TM form (no fees). They will return both after processing time of 5 to 20 minutes (depending on how busy they are).
TIPS
Total time lapse between arrival at the border and crossing back into Thailand: 30 to 90 minutes. bullet
Driving time from Pattaya to Aranyaprathet: 4 to 6 hours.