Identify the element whose oxidation number you need
Assign oxidation numbers to known elements using standard rules
Use the sum of oxidation numbers in the compound or ion
Set the total equal to the overall charge of the species
Solve for the unknown oxidation number
Check that the result matches common oxidation state rules
Remember that elemental forms have oxidation number 0
Remember that monatomic ions have oxidation number equal to their charge
Remember that oxygen is usually -2
Remember that hydrogen is usually +1
Remember that Group 1 metals are usually +1
Remember that Group 2 metals are usually +2
Remember that fluorine is always -1
Remember that the sum of oxidation numbers in a neutral compound is 0
Remember that the sum of oxidation numbers in a polyatomic ion equals the ion charge