For example let’s pick 6 and 12. To find the greatest common factor, you will break down the number into the numbers you can multiply to get 6 or 12 as much as you can until you reach a prime number.
Now to find the least common multiple, you can set two tables, one of the multiples of 6 and another of the multiples of 12 and see what numbers they have in common and select the smallest number of the multiples they have in common.