you find the LCM of the denominators in the fractions, then you multiply both fractions as needed till both have like denominators. Then you subtract.
For example:
5/9 - 2/4
LCM of 9 and 4 is 36
[(5 x 4)/(9 x 4)] - [(2 x 9) - (4 x 9)]
= 20/36 - 18/36
20/36 - 18/36 = 2/36
2/36 = 1/18
hope this helps ya!