Well let's take something like combustion of methane: reaction of methane with oxygen to form water and carbon dioxide.
What is the formal charge on each atom before the reaction and after? E.g., look at the oxygen atoms before the reaction (i.e., O2 gas) and after the reaction (in water and carbon dioxide). Does the formal charge of oxygen change? Can you find any other atoms that formally change charge?
You should then write a balanced equation for the reaction and convince yourself that all the formal charge changes balance out to zero.