Answer:
It is very easy. Most of the time all you have to do is multiply the two denominators by each other, then multiply the numerators by the opposite numerator.
Explanation:
For example, if you have 1/3 and 1/4. The common denominator of both is 12, because 4 times 3 = 12. Then you have to multiply the numerators, so it would look like this in the end: 4/12 and 3/12. 4/12 when simplified is 1/3, and 3/12 when simplified is 1/4.