To convert Arabic numeral (Western numeral i.e. everyday numbers) you have to separate number into thousands, hundreds, tens and units. With Conventional Roman numerals you can express numbers from 1 to 3999. There are other alternative forms where you can express numbers greater than 3999.
Splitting 300 we have: 300 => CCC. We have to concatenate corresponding roman numerals from thousands to units i.e. CCC. So the final result is CCC
