Here are some examples of correctly using the serial comma:
The flag is red, white, and blue.
(Three predicate adjectives.)
In her will, the woman left jewelry, coins, stocks and bonds, but no cash.
(Four direct objects of the transitive verb left.)
The director, the assistant chief, and the chairperson held a confidential meeting.
Neither the president, the vice-president, nor the chief financial officer may authorize this particular capital expense.
The personnel committee reconsidered this issue, found that the supervisor had exceeded her authority, and granted the relief requested by the employee.
(Three predicate verbs.)