I have opened a few die necks to make my own forming sets and it's fairly easy.
Size dies are case hardened. It takes carbide to cut it. I used cheap "concrete" carbide drill bits I first ground to the diameter I wanted. It takes a diamond wheel to grind the carbide.
The hardest part is to feed the bit into the base of the die centered. My drilling was done on an old 9" South Bend lathe, bit in the tailstock and die in a 4 jaw chuck.
It's best to grind the bit a few thou undersize and plan to lap the rest out to diameter because the make-shift bit will score the metal as it cuts. Lapping should also be done on the lathe and that will remove most of the score marks.
Obviously, the best I can do is less precise than a major maker can do but for intermediate dies to be used in case refoming, it is close enough. I have forming dies for taking .35 Whelen/.30-06 down to 7-08, .260, .243, .250 Sav., .22-250 and 6 mm International, all done this way. (I had a small pile of sizing dies I got cheap at flea markets, yard sales, ebay, etc. as donors for this effort.)