This step is pretty simple. You want to cover up the shallow plastic storage tub you use with as much of the plastic bread rack tray as you need. You won't be needing the lid of the plastic storage tub.
Since bread rack trays (at least in this area) are 24.5" X 24.5" square, I was lucky enough to find a plastic storage tub (designed to slide under a bed) that was approximately 24" long by approximately 16 1/2" wide by 4" deep.
Since you don't want your dog to relieve him/herself on any part of the mesh rack except over the tub that will collect the urine underneath, you'll want to cut the plastic bread rack tray so that it fits over the plastic storage tub below.
Using a Dremel (rotary) cutter, I trimmed approximately 7" off one side of the bread rack tray so that it would slightly overhang the edge of the plastic storage tub. The remainder of the bread rack tray fit perfectly over the plastic storage tub.