Learned them in grade school, about an hour's learning session and an hour's practice. Didn't really forget them but about a month ago realized I had a significant Roman numeral on my license plate.
I guess for a zero, you just use " "...
Mull, mull, think, think....
"How many .38 caliber revolvers do you own?"
"I dunno, maybe III or IV. VI if you count my II IXmms as XXXVIIIs."
"OK, just wondered. How many fifty-caliber machine guns do you own?"
" ."
Terry