Instructions
Learn what values the letters represent. I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1000. There is no zero.
Learn the rules for putting letters together. Two letters of equal value next to each other are added: II is 2, XX is 20. If a letter of greater value comes before one of lesser, they are also added: VI is 6, XI is 11, DCLI is 651.
If a letter of lesser value comes before one of greater, it is subtracted. IV is 4, IX is 9, CM is 900. Numbers are always written in the simplest possible form, meaning 4 will always be IV not IIII and 499 CDXCIX. Notice how the math is done in chunks of two letters.
Break dates down into place values to easily convert them. For example 1988 is 1,000 (M) plus 900 (CM) plus 80 (LXXX) plus 8 (VIII). Put these together, and 1988 is MCMLXXXVIII.