There is a long bolt threaded the entire length. His collar was not aluminum but I'm sure it was similar in size - basically a cylinder with an open top and a hole in the bottom for the bolt to fit through. The open end of the cylinder is designed to fit over the lip of the opening in the cross member where the bushing is. A big heavy duty washer and nut screw onto the bolt on the back side of the collar - pretty sure it was much thicker and bigger than one of the front differential washers.

Once you remove the differential, you slip the bolt through the bushing and fit the collar onto the cross member. Then on the opposite side of the bushing, you can slip on the piece that will press the bushing. His looked aluminum, fairly thick (>1"), and was stepped so that one side diameter was about 1/2" larger than the other side (I assume it was repurposed). The larger side hopefully fits the hole perfectly to press the diff bushing. The tool was designed for a 350Z, and I vaguely remember several years ago reading about the Whiteline 350Z bushing needing to be shaved down for application in a 370Z. Lastly, of course, on the back side of the press piece is another heavy duty washer and nut. Cinch everything up tight and then tighten down the nut behind the collar to pull the bushing into the collar.