I haven't taken that one, but I have used the ferry from Vidin (BG) to Calafat in Romania.
We just parked our car in line (like the locals), then strolled to the ticket office and bought the trip across the Danube.
After that, followed the rest of the vehicles up the ramp and onto the 'ferry' (more of a floating barge) and 20 minutes later we were in Romania.
No hassle or stress!
Don't forget to buy the appropriate 'vignette' when you land at either side (you can normally do this before you hit the roads)