Memorize the basic numeral representations for Roman numerals, which use letters to represent numbers. The letter "I" represents 1; the letter "V," 5; the letter "X," 10; the letter "L," 50; the letter "C," 100; the letter "D," 500; and the letter "M," 1,000.
Follow basic rules to combine letters into more complex numeral representations. One important rule is to never repeat the same letter more than three times in a row. For example, III is 3; XXX is 30; and MMM is 3,000.
Apply an alternative letter arrangement instead of using the same letter four times. This involves placing a letter of a lesser value in front of a letter of a larger value. If you see that arrangement, you must subtract the smaller value from the larger value. For example, the number 4 is written as IV because you're not allowed to write IIII. IV stands for 5 - 1, or 4. The number 40 is written as XL since you're not allowed to write XXXX. XL means 50 - 10, or 40.
Recognize that the smaller-value letter placed before the larger-value letter can only be one-tenth or one-fifth the value of the larger letter. For example, you're not allowed to write the number 99 as IC. The problem here is that 1 is 1/100 of 100, which isn't allowed. So you must form the 90, followed by the 9. The 90 part is XC because you can't write LXXXX. The 9 part is IX because you can't write VIIII. Combining these parts, you get XCIX for 99. Notice how in both cases, the smaller letter is 1/10 the value of the larger letter, which is allowed. Note that IV is allowed because I is 1/5 the value of V. CD would also be allowed to represent 400, since C is 1/5 the value of D.