Taking a flight is the fastest and easiest way of getting from one country to another. It is also the most expensive. If you are traveling for a longer period of time you try to save up as much as you can so you could extend your trip. For some, it terrifies them to do land border crossing. But it is actually super easy.
Bus tickets can be purchased online, in the bus station, in travel agencies in Cambodia or you can ask the receptionist in your hostel/hotel because most are affiliated with bus companies. If you have plenty of time and prefer the traditional method of doing things, you can always visit their bus station and book your trip there.
Most of the buses offer free pick-up to hotels/hostels affiliated with them. You can see the list of hotels/hostels when you book your ticket online.
In the terms and conditions part of your ticket, it says that you should be ready for pick-up one hour before the trip. The bus has a compartment where you can put your luggage. You will be provided with a luggage tag if you place your bag there. You need to keep this tag because they will check it when you retrieve your bag.
The trip from Siem Reap to Bangkok costs around USD 32 and USD 1 credit card processing fee. Bottled water, a pack of facial wipes, snacks and packed lunch are provided. They also hand out an immigration card to fill out. The bus attendant assists in filling out the form in case you have questions. The bus travel from Siem Reap, Cambodia to Bangkok, Thailand takes 9 hours.
You'll reach the border crossing station at Poipet after three hours of land travel. You will be asked to alight the bus and walk to the Cambodian immigration.
At the Cambodian Immigration, you line up like what you normally do at any immigration. You give your passport to the immigration officer, have your picture and fingerprints taken. Then you wait for the immigration officer to stamp on your passport. The immigration card you filled in at the bus is for the Thailand Immigration. They don't ask for it at the Cambodian Immigration.
Passing the immigration at Poipet is easy, it takes around 10-15 minutes including waiting in line. From the border crossing station, you will then be told by the bus attendant to wait in front of Tropicana Resort and Casino. When everyone from the bus has completed the process at the Cambodian Immigration, you will walk together to Thailand Immigration.
The line at the Thailand Immigration is very long. It may take around one hour to finish. But the process is basically the same. You hand your passport and the immigration card, have your picture and fingerprint taken and then wait for the immigration officer to stamp on your passport.
After passing the Thailand Immigration, you will walk back to the bus and there you will be given the packed lunch. You can eat your lunch while waiting for the others to return. After that, you will continue your trip to Bangkok. In Bangkok, you will be dropped off near Khao San Road.
Hope this post helped you with your queries regarding border crossing from Cambodia to Thailand.