Use liquid water to paste a paper template to the slab. I thought that a paintbrush would be good for this, but it turned out that just dribbling water on the paper was easier. A spray bottle might work well too.
Following the template, cut out the gear blank. I used a hand pruning saw which is good at cutting ice but not particularly precise. Ideally, the cut would be perfectly perpendicular to the surface of the slab and exactly following the outer circle on the template. Use a 1/4 inch spade bit to drill the center hole and two holes for the sprocket assembly.
Mount the gear blank on the blank spindle. It should be seated on the sprocket assembly so that they turn together. Place the top spindle bar through the bar rails and onto the spindles. Now when you manually move the spindle bars from side to side, the guide rack and pinion cause the sprockets on all three spindles, as well as the gear blank, to rotate. At this point, you will probably need to adjust the guide rack position to the left or right in order to allow your first cut to follow the template. Your first cut should be with the gear spindle to the left of the saw blade.
With your left hand, provide a little clockwise tension on the gear blank. Make your first cut with the circular saw by pulling it down through the gear blank and then back up until the saw blade is clear of the blank. Move the rack to the right (you'll soon get the hang of how far), provide clockwise tension, and make your next cut. This process allows successive cuts to follow the involute tooth profile.
After finishing each tooth, manually advance the guide pinion with respect to the guide rack to set up the next tooth for cutting. For this gear, I advanced two guide pinion teeth for every one tooth on the gear blank.