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 353 we have: 300 => CCC, 50 => L, 3 => III. We have to concatenate corresponding roman numerals from thousands to units i.e. CCC, L, III. So the final result is CCCLIII
