For an e-visa, an online form needs to be filled and submitted, along with the fee.
For a traditional tourist visa, the following will be required:
Original, signed passport valid for 6 months beyond stay in Cambodia, with at least one blank passport page available for Cambodian visa stamp.
One signed Cambodia visa application form plus one copy.
One recent 2 x 2 passport type photograph, in color, front view and with a plain/light background.
Computer generated flight itinerary showing round trip travel to Cambodia and onward flight.