By opening the car door and stepping out (very carefully) onto the edge of the ramp and then walking down it.
I spent 2½ years working as a 'trade plater', delivering and collecting cars from major depots. I watched many transporter drivers going about there work and was often amazed by how they did it. Getting down the ramp is the easy bit. Adjusting the decks, to fit the maximum number of cars into a finite space, is far trickier. It's easy to end up crushing the roof of a car on the lower deck if you're not careful (and I've seen it done more than once!).