Step 2: Opening up the ping-pong balls
Drill a hole well inside the marked circle, as a starting point for cutting out the marked area with curved scissors. Cut it out slightly small, to finish it with sanding paper: first put it on flat sanding paper on your work surface and sand by rotating it. Do this till the outside of the edge is smooth. Then make a roll out of the sanding paper and put it in the hole. Smoothen the inside of the edge by rotating the roll.
This usually takes only a couple of minutes, as the material the pin-pong balls are made out of is quite soft. The resulting diameter of the hole is not critical. It can deviate a couple of mm.