The process is explained in detail in the Handloaders Manual of Cartridge convertions. I believe the author has a custom made contraption for this purpose. I will try to find the chapter and page later.
PW

I found it. Its on page 50 of the before mentioned manual
PW

I believe that is where I saw it mentioned. I didn't read far enough.I have fireformed cases in rifle chambers using bullets and somewhat reduced loads, and can see where the cornmeal/wax plug would be cheaper. But it looks like it would also be messy. I will read the article. Thanks.