@amarand Very carefully. π While it's still wet, but not too squishy, I used several things to round it. The opening of an old glass milk bottle, the opening of a small glass vase.
In the end, the right thing for the size was just an empty plastic pill bottle from the pharmacy. I gently rolled the ball on the opening of the bottle. It shaved off any extra dirt.
@amarand It looks like people sell them on Etsy for around $50-$100. I'd say they need to be packaged well for shipping, though. It's dry mud. The outer layer is the most fragile.