The steps for subtracting fractions is well illustrated in the fraction calculator at the link below. That page has a more detailed discussion of common denominators, mixed fractions and other topics that may help understand the steps necessary to subtract fractions.
Practice with worksheets. Once you have mastered the flash cards, try your hand at some worksheets. Start by practicing with one number set at a time. When you have mastered all of them, try worksheets that mix up all of the number sets and see how you do. If you know your tests will be timed in school, try practicing with a timer.