The first thing to know is the value of the Roman digit. These are given below.

1=I
5=V
10=X
50=L
100=C
500=D
1000=M

Now the only thing to write Roman numbers are the logic required.
Right side of bigger digit of Roman system is added, and the left digit is subtracted.

READ :  Roman Number Leaning for Class 3 | Very nice video with Examples