Youll need a course with at least one existing grade item in it. Go to the gradebook to the categories and items screen. Add a new grade item (theres a button beneath the categories and items table). Once its created click the calculator icon in the new grade item's row. In the ID Numbers section enter some names for the existing grade items. For example, firstitem and otherassignment. Check that they save without error. Enter a formula using the ID numbers. for example =[ [firstitem] ]+[ [otherassignment] ] Check that it saves without error. It will complain about an invalid formula if it's not correct.