Subtracting fractions is similar to adding fractions. Most of the work involves the denominator. If the denominators of the two fractions are already the same, much of the work is done. Simply subtract the two numerators to get the answer fraction using the same denominator, and then reduce and make a proper fraction.
In KS2 children are taught that decimals are another way of writing fractions. The hundred number square is a really good way of showing children the equivalence between fractions and decimals.