Start dividing by prime numbers. Since the number is even two won't work so next is three. If you divide 429 by 3 you get 143.
You continue doing this with primes going up (5, 7, 11, 13, etc.) until you get to the final prime. 5 and 7 don't work if you try dividing them by 143 individually so next up is 11. If you divide 11 by 143 you get 13 which is also a prime number.
