When I had to solve this problem, I created a hard-coded data dictionary to map between numbers and their associated words. For example, the following might represent a few entries in the dictionary:
{1, "one"}
{2, "two"}
{30, "thirty"}
You really only need to worry about mapping numbers in the 10^0 (1,2,3, etc.) and 10^1 (10,20,30) positions because once you get to 100, you simply have to know when to use words like hundred, thousand, million, etc. in combination with your map. For example, when you have a number like 3,240,123, you get: three million two hundred forty thousand one hundred twenty three.
After you build your map, you need to work through each digit in your number and figure out the appropriate nomenclature to go with it.