Select a prime factor of your starting number and divide the number by the prime number. For example, if you start with 42, 7 is a prime factor, so divide 42 by 7 to get 6.
Repeat Step 1 until no more prime factors exist. In this example, divide 6 by 3 to get 2. Since 2 is a prime number, no more prime factors exist.
Write the prime factorization as each of the prime factors multiplied by each other. In this example, because the prime factors are 7, 3 and 2, the prime factorization is 7 x 3 x 2.