I have done a lot of these, and have done them a couple of ways. Mine
are only slightly domed, note, not fully hemispherical.
First I sawed the discs out then planished the centre area gently til
the disc domed up. The discs retained the planished surface which I
accepted as part of the design.
Then many years later I hammered each disc down into a wooden hole I
had made by a local woodworker (retired). Used a fat delryn punch.
Now I use a custom disc-cutter and dome 3 or 4 at a time with a press
using a polished dome tool and a red lurethane block encased around
the edges with steel (so the power of the press mainly works on the
Pop 'em in the tumbler.
Auckland NEW ZEALAND