You can follow thisif im not.mistaken
I=1
V=5
X=10
L=50
C=100
M=500
D=1000
So if you will write 126
It is 100 + 10 + 10 + 5+ 1
C + X + X + V + I
CXXVI
but if the num is 90
Then it has be written XC
the roman numerals must be on descending order
From highest to lowest but if some are not you will diminished the first to the next
Like on the 90 written as Xc the first is the x so you will diminished 10 to 100