Be sure that the sum equals the ionic charge but then assign values that make the total oxidation change easy to calculate. This may lead to strange values, but can be an aid to balancing the equation. You will also need to practice working with fractions. Note that this is the exact total oxidation change for this compound you must get no matter how you assign the numbers to the elements. Rules for assigning conventional oxidation numbers.
Uncombined elements have an oxidation number of zero.
F is always -1 in its compounds. For metallic elements, you might start by assigning a number equal to the element's periodic column number using the Roman numeral system.