List all factors of each number
Identify the common factors between the numbers
Multiply the common prime factors (each shared factor used the least number of times)
If using prime factorization: write each number as a product of primes, then take only the primes that appear in all numbers with the smallest exponent
If using factor lists: the GCF is the greatest number that appears in every list of factors
For two numbers: GCF is the largest common factor
For more than two numbers: GCF is the largest factor common to all numbers