When making the pistons, I mounted them on their rods for finishing. This ensured that they were concentric with the rods. Here, you can see that I've stuck the rod in a collet for turning the ring grooves.

The tool is ground to the width of the rings to ensure that there's no slop in the groove. Depth of the groove is less critical, but I was still pretty careful about it.

Two grooves per piston. (By the way, if you make your own rings, make twice as many as you need. You should plan to break some, and you'll likely need some for heat treating experiments and whatnot until you get the whole process figured out.)