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.
If you make your own, you can just draw three circles on a page and then 2-5 triangles on a page, and ask the child to match each circle with a triangle by drawing a line from shape to shape. Vary the shapes and the amounts. Sometimes the amounts should be equal, sometimes not.