I made this quick mockup to show how I intepreted it. Someone please step in and correct me if you see a potential problem.
The top image shows the two fixed pieces for the jig. The top piece has a groove routed in it to fit your bushing of choice. Set the length to be the longest slot you want to make. One edge of the groove gets a screw in it which acts as a sort of stop (thats the pink thing in my render). Not sure if its just a screw into the wood, or if a threaded metal insert was used (that's what I will do).
Then the bottom image shows what it would be like on a workbench with a piece of thin stock in it (the blue piece). Notice how the additional 3rd part of the jig gets positioned and clamped (clamps not shown obviously) in order to set how far from the edge of the workpiece the hole will go.
That's about all the video shows. It also shows how he makes several passes with the router at increasing depths instead of just one deep pass, but that's just common router sense. Also he had lines marked on the top piece, perpendicular to the groove to help with alignment. One line seemed to line up with the "minimum" position of the adjustable screw (as in, with the screw unscrewed to make the smallest slot). I am guessing he continued some of those marks on the vertical piece under the slot as well to aid in positioning the stop.
Hope it helps.