X * III + I
It's actually not too complicated. For addition, you just stick all the letters together and then fix up cases where it breaks the rules (e.g. IIII -> IV). For subtraction, you take out matching letters, and you can replace a letter with multiple copies of a smaller letter (e.g X -> VV) if you need to.
Multiplication is a bunch of replacements. You multiply the whole first number by each letter in the second number, so X * III is X, X, and X, which you stick together to be XXX. Once you're done with that, you go do the same fixup that you had to do for addition. You only have to memorize a fairly small times table (V * V = XV, for example).
Mega Man: The Light of Will (Mega Man / Green Lantern crossover: In the lead-up to the events of Mega Man 2, Dr. Wily has discovered emotional light technology. How will his creations change how humankind thinks about artificial intelligence? Sadly abandoned. Sufficient Velocity x-post)
Games by Coda (updated 11/24/2019 - New game: Jigsawmino)
Art by Coda (updated 4/20/2020 - untitled original music)