Before backing the second trailer onto a converter dolly to couple it, what must be done with the dolly and trailer?

When coupling the rear trailer to a converter dolly, the dolly must be kept from rolling — chock its wheels or otherwise secure it — so it does not shoot out from under the trailer or move during the hookup. The dolly is positioned so its fifth wheel lines up under the kingpin of the trailer to be coupled, with the fifth wheel tilted down at the right angle. The trailer must be supported at the proper height so it can settle onto the fifth wheel without dropping. After backing under and locking the fifth wheel, the driver checks that the kingpin is fully engaged and that there is no gap between the trailer and the fifth wheel.