How do you use Roman numerals?

Answer 1

Simply use certain letters to represent numbers, e.g.
1=I
2=II
4=IV
5=V
10=X
50=L
100=C
500=D
1000=M

However, judging from that, you may have guessed its a little more compicated than the heading would suggest, but, its still fairly simple.

IV is 4 because the I (1) is before the V (5), and so it basically represents 5-1 which is 4
VI would be 6 because the I is after the V (5) making it essentially 5+1 which is 6
The same applies with X (10)
To write 9, simply take the X and then put an I before it, making the number 1 less than 10, which is 9
The same can be used for II and III to make 12 and 13 as well as 11. e.g XII = 12
A number like 8 is not IIX because this is represented by VIII (5+3)
Other Roman Numerals can be used in a similar way, e.g: 600 = DC, because here the C (representing 100) is after the D (representing the 500) and so the two represented numbers are added
400 would be CD because the C (100) is before the D (500) and so C is the amount taken away from D making 400.
Any number can be written with this system,
2009 = MMIX (two 1000s added together with a IX at the end which represents 10 take 1 (9) Two thousand and 9.

1356 would be MCCCLVI ( M, which is a thousand, with 3 Cs (300) added to that, making it 1300, then a 50 is added (L) followed by a 6 (VI), thus making One thousand three hundred and fifty six.
If you still do not fully understand how it works, or want to know more, here is a site to help:
www.novaroma.org/via_romana/numbers.html

Answer 2

The Romans used "Roman Numerals" the same way that we use our Arabic numerals. They were used for all things mathematical, accounting, adding, subtracting, etc. The difference was just in the writing of the numbers, such as V for a 5 or X for a 10.