Cambodia is another fun destination in South East Asia. The best part is that most people can get a Cambodia visa at the border posts. All you need to do is to fill a visa form and pay a fee of 20 USD. You need to fill in an address in Cambodia . Fill in the hotel you plan to stay at. You will need a photo too.
One of the bus plying Cambodia -Vietnam route
This is the process at Bavet (Cambodia) MocBai (Vietnam) border (this is the usual bus route from Saigon to Phnom Penh) but the process is similar at Thailand-Cambodia border too. Once you have finished Vietnamese exit process the bus will take you a little distance away to the Cambodian immigration. There is a visa on arrival counter where you can fill the form and pay the fees. The officer will paste the visa. After this you can join the immigration queue.
Cambodia Arrival Form
This is how the arrival card looks like. You will have to fill in the address in Cambodia even if you are not sure what hotel you are staying at. Just fill in the name the hotel you intend to stay at.
You can also apply for Cambodian e-visa, this is by far the most convenient method though you need a lead time of 2-3 days ( I have gotten the e-visa in 24 hours most of the time). I have often heard of travelers being hassled for more money at Cambodian Thailand border. If you have the e-visa, you just walk through and you don't have to pay anything extra. You apply via the e-visa website here. http://www.mfaic.gov.kh/evisa/ and these are land borders where you can use e-visa http://www.mfaic.gov.kh/evisa/Map.aspx