Faced with a blank editor and a problem, the instinct is to start typing. Programmers who solve hard problems reliably do the opposite:

Understand the problem. Work a few examples out by hand, including awkward ones.
Plan the steps in plain English, as comments.
Split the plan into small functions, each with one job.
Build and test one function at a time, smallest first.

Here's the plan for converting a number to Roman numerals. Working examples by hand first (1994 is MCMXCIV) shows the key idea: take the biggest numeral value that fits, write it, subtract it, repeat. The awkward cases like 4 (IV) and 900 (CM) are easiest handled by treating them as numerals in their own right.

# to_roman(n):
#   list the numerals from biggest to smallest, including IV, IX, XL, ...
#   for each numeral:
#       while it still fits into n:
#           add its letters to the answer
#           subtract its value from n
#   return the answer

A plan like this turns into code almost line by line.