Write down the two fractions. For example, you might have 1/3 and 3/4.
Select a number that both denominators will divide into evenly. Usually, it's better to choose the "least common denominator" to save yourself some work later. In this example, the lowest number that both 3 and 4 will fit into evenly is 12.
Place the least common denominator in the denominator position of both fractions.
Multiply each numerator by the quotient of the least common denominator divided by that fraction's original denominator. In this example, 12 divided by 3 is 4. Multiply 4 times 1 to get 4. Therefore, the first fraction would be 4/12. In the second fraction, 12 divided by 4 is 3. Multiply 3 by 3 to get 9. This makes the second fraction 9/12.