Set an oxidation number for every element and watch the total chase the real charge. Then step through a redox equation and see which element climbed and which one fell.
Remember the order. Fluorine, then group 1 and group 2 metals, then hydrogen, then oxygen. Whatever is left you solve for, using the rule that the numbers add up to the charge.
What to watch. One element always climbs and another always falls. The electrons lost must equal the electrons gained.