Although within the Roman Empire itself it wasn't enforced a set of stricter rules that would have lead to the standardization of Roman numerals writing, in the last hundred years some rules were applied to eliminate further confusion.
We concluded that there is a set of six rules to be remembered about when writing numbers with Roman numerals.
Read them below, in order. Don't worry if you just don't understand something right away. When you finish reading all the six rules, things shall become clearer.