Haven't done it since December 2019 (second time), but it always used to be like this:
You'll have to pay for a Cambodian visa (on arrival) - can't remember how much, but I seem to recall that paying in dollars was a better deal than baht.
(Think it was $40 US... or 1,500 baht)
If you do a turn around they take money off you - again, can't remember, could have been a 300 baht 'fee' if you don't stay.
The Cambodian immigration guys are pretty po-faced, gruff and unfriendly.
Also. they may try to take that fee twice - once on entry when you tell them you're not staying anywhere (they'll ask), and again when you visit the window to exit. So keep receipts for payment, and don't let them get away with that scam.
I assume that you're not thinking of taking your car in, so you'll need to find somewhere to park as near as possible on the Thailand side, and that wasn't so easy when I did it... you may need to look around.
Altogether it's not such a bad experience as such things go... may take a couple of hours including a short break for something to eat/drink on the Cambodia side.... although less busy now I imagine.
(The immediate vicinity of the town is a dusty, horrible dump.... you won't be tempted to hang around.)
I drove from Pattaya, so it was a long day with the drive there and back, making me more tired and less patient than I might otherwise have been.
Good Luck!