How to convert and write Hindu-Arabic numbers in Roman numerals: breaking down into place value subgroups (units, tens, hundreds, thousands, etc.), in expanded notation
Arabic numbers decomposing (breaking down) in place value subgroups before converting to Roman numerals:
To turn any Arabic number into a Roman numeral, break down that Arabic number (decompose it) into place value subgroups, in expanded notation, of units, tens, hundreds, thousands, ten of thousands, hundred of thousands, millions, etc..
Convert each of these subgroups separately to numbers written down with Roman numerals;
Wrap it up: catenate these subgroups using the additive notation to get the end Roman numeral.