The oxidation number is the charge an atom would have if all bonds were completely ionic
Free elements have oxidation number 0
Monatomic ions have oxidation number equal to their charge
The sum of oxidation numbers in a neutral compound is 0
The sum of oxidation numbers in a polyatomic ion equals the ion charge
Group 1 metals are usually +1
Group 2 metals are usually +2
Fluorine is always -1
Oxygen is usually -2
Hydrogen is usually +1
In metal hydrides, hydrogen is -1
Halogens are usually -1 unless combined with oxygen or a more electronegative halogen
Assign known oxidation numbers first
Use the total charge to find the unknown oxidation number
Check that the final values add up correctly