Memorize the Roman numerals. I means 1), V represents 5) and X is used for 10). L represents 50, C stands for 100), D means 500 and M represents 1,000. The mnemonic phrase "My Dear Cousin Loves eXtra Vitamins" is helpful because its capital letters -- M, D, C, L, X and V -- are Roman numerals that mean the same as the Arabic numerals 1,000, 500, 100, 50, 10 and 5, respectively. A numeral with a line over its top means to multiply it by 1,000.
Look at the first two Roman numerals in a long Roman numeral you want to read. If a numeral appears to the right of a numeral that is of equal or higher value, then add those numerals. For example, MDCLXVI = 1000 + 500 + 100 + 50 + 10 + 5 + 1, which equals 1666.
Subtract the numeral on the left from the numeral to its right when a smaller Roman numeral appears to the left of a numeral of higher value. For example, MMMCMXCIX translates to 3,999.