You first list out all the factors.
For example, 25 and 30.
25: 1, 5, 25
30: 1, 2, 3, 5, 6, 10, 15, 30
Second, you look at all the common factors.
The common factors are: 1, and 5.
Finally, you take the greatest common factor.
That would be 5. So, the GCF of 25 and 30 is 5.