How do you find the highest common factor of two numbers?

Lets take 18 and 48 as our two numbers for an example.
Break both down into products of their prime factors using factor trees: 18=3x3x3x2 and 48=3x2x2x2x2.
Write the prime factors as a Venn diagram. The numbers in the intersect (in the middle) are 3x2 which means the highest common factor is 6.
The way to find the highest common factor of numbers is by writing them as a product of their prime factors and then looking at the intersect when written in a Venn diagram.