Draw a circle on a piece of paper or two, depending on the size of the fractions to be compared. The slices don't need to be precise as long as the number is clear.
Divide the circle into the relevant size for the fractions: quarters, fifths, tenths and so forth. If possible simplify the fractions or find a common denominator. If the fractions share a common denominator than the fraction with the larger numerator is larger.
For example: when comparing 8/20 and 3/10, 8/20 can be reduced to 4/10 by dividing both sides of the fraction by 2. 4/10 can be reduced further, but leaving it in tenths makes it easier to compare with 3/10.
Fill in the circle with the amount in each fraction.
For example: 4/10 means filling in four of the ten spaces within the pie, while 3/10 means filling in three slices. There should be three slices left empty.
Compare the two fractions within the pie. Since 4/10 takes up more of the pie, it is the larger fraction.