Using a lathe.
Looks like having access to a good lathe is absolutely necessary. He takes a raw puck of aluminum, drills out the center, then cuts in from the outer edge until it reaches the size needed.
After using the lathe to machine a blank exactly the same size as the existing bezel insert, Nick then paints the insert black with some simple spray paint. I think the next step is the coolest:
Using a cheap laser cutter (something like this one) he etches the markings directly into the aluminum.
And then just a bit of clear coat spray paint to finish off the insert.
Not too bad looking eh? If you’re curious about the numbering on the insert it’s designed to allow either timing by the minute or by the hour.