Dividing the top and bottom by the same number keeps the amount the same. It is Equivalent Fractions worked backward, with fewer, larger pieces. You are finished when the only number that divides both is 1.
eight twelfths
Both are even, so divide the top and bottom by 2: 8 ÷ 2 = 4 and 12 ÷ 2 = 6. The amount is the same, but 4 and 6 still share a 2.
Divide by 2 again. Only 1 divides both 2 and 3, so this is simplest form.