I've never seen a detailed description of the techniques used for this.
What kind of detail are you looking for? Assuming it's not a figured blank (one in which some of the final shape is already molded), it would have been entirely carved with wheels - probably stone, diamond and/or copper with carborundrum and water for grit. Stone and diamond remove more material than copper, and would have been used for the general shape. Copper might not have been used at all.
I have a video from the Corning Museum about a Czech engraver and I might be able to find out more precisely the details, but it would be a couple weeks probably before I could get back to write about it, as I'm going backpacking in southern California in a couple days and don't have time to watch it.