Reduce the fraction by the greatest common multiplier. For example, if the fraction is 5/20, then 5 is the greatest common multiplier. The fraction would reduce to 1/4.
Write the numerator first. The numerator is the top number of the fraction. From the previous example, this would be 1. Then write a colon next to the number.
Write down the denominator after the colon. The bottom number of the fraction is the denominator. Your final work will look like this: 1:5.