The Rules for Writing Large Numbers in Words
Large numbers are written in words by breaking them into groups of three digits from the right (matching the comma-separated groups in standard numeral notation), naming each group of three using the same basic hundred/tens/ones rules, and appending the appropriate scale word (thousand, million, billion, trillion) to every group except the last — so 4,502,190 becomes "four million, five hundred two thousand, one hundred ninety."
Once this grouping pattern is clear, spelling out even a very large number becomes a mechanical, repeatable process rather than something to work out from scratch each time.
Breaking a number into its three-digit groups
Each group of three digits (thousands, millions, billions, and so on) is spelled out using the same internal rules — hundreds place, then tens and ones — and then labeled with its scale word; only the final, smallest group of three (the ones-hundreds-tens closest to the end) has no scale word attached, since it represents the base value itself.
A worked multi-group example
For 12,004,300: the group "12" gets the label million, the group "004" (just four, since leading zeros in a group are skipped) gets the label thousand, and the final group "300" needs no label — giving "twelve million, four thousand, three hundred."