Many travelers especially Europeans and North Americans journey to this side of the planet, armed only with their Lonely Planet handbooks, which to be honest, does not give you a very realistic account of what happens in the border.
Khao San Road to Siem-Reap
But before that, the first advise I can give you is to not pay more
than 300 baht for a one-way trip from Khao San Road to Siem Reap. A lot of travel agencies along the street will try to charge you 400 to 600 baht and if it you first time to take the trip, you might fall for it.
I recommend you to visit travel agencies along Soi Rambuttri, a street parallel to Khao San Road, which you can reach by going through the Susie Walking Street passageway.
With 300 baht, I can guarantee you that you will have a nice van along with other travelers. The trip is roughly about 4-5 hours depending on the traffic and how fast your driver is. Some vans take more stops in stores and restaurants than necessary and if you are smart, you will notice that these drivers are friends with the store owners and I think they get some sort of a commission for every purchase you make at the store.
Cambodian Visa
Then, just when you think that you will finally be able to cross the border, the van stops yet again at a restaurant and there you will be greeted by someone who claims to work for the Cambodian Immigration, armed with large IDs.
They will then try to charge you $40 to $45 for the visa and will charge you more if you don't have a photo. They claim that those, who refuse this fee, will not be permitted to enter Cambodia.
On my very first border-crossing, I have experienced the same thing and I thought that I was really supposed to pay for the visa along with everybody else but a fellow Filipino, warned me not to pay since as it turned out, Filipinos do not need visa to enter Cambodia.
I felt relieved but I felt sorry for friends I have met along the way who had to pay the $40 to $45 visa. But I was really dumbfounded and heartbroken when I learned that the visa actually, only costs $20 and you can easily get it at the Cambodian entry-point in as fast as five minutes.
Leaving Thailand, you will get you exit stamp at the town of Aranyaprathet and will walk several blocks into the gate of the Poipet entry in Cambodia.
Poipet to Siem Reap
As soon as you get that entry stamp in Poipet, you will be guided to the government transfer bus right at the back of the immigration to the main bus terminal. The transfer bus is free and you don't have to pay anything. The transfer takes about 30 minutes.
At the main terminal, you will take another bus ride which will take you to the heart of Siem Reap after two hours of road trip.
Take note that the bus, will probably stop at a specific hotel, which I suspect if their partner hotel.
Tired and weary of the long ride and numerous transfers, some travelers are convinced to stay at the hotel but I advice you to just take a tuk tuk and ask the driver to the main street and look around for an affordable hotel.
A tip: ask the tuk-tuk driver to take you near Pub Street and there are numerous hotels, inns and hostels around the area.
Currency Exchange
Some touts including those who claim to work for the immigration might take you to a nearby money exchange center right before border crossing convincing you that Thai Baht and US dollars, are not readily accepted in Siem Reap. You
Again, this is a lie. You can easily use both currencies in any store and restaurant but for change, they will give you Cambodian riels. And as you will discover soon enough, the exchange rate at the border crossing is different that that in Siem Reap so don't be fooled.
I hope that these tips will help you navigate through the border. And if you indeed experience these kind of experience, try not to get angry but insist in doing it your way. As soon as you reach the immigration office, it is pretty much straight forward and you can easily secure the visa.
And please pass the info to anyone of you friends who intend to cross the border too as a kind of traveler's compassion.