How do I file for a divorce by placing a ad in the newspaper?

You are confused. You don't file for a divorce by placing an ad in a newspaper. You file for divorce at the Clerk of Court's Office at the local courthouse. You then place an ad in the local paper for so many insertions saying certain wording. The Clerk will help you with this wording and advise you of how many times it must be advertised over a certain period.

Correct, you file for divorce by filing the motion for a divorce at the courthouse and paying a filing fee.

If you have no idea where the other party is, the court will have requirements for posting in certain newspapers a legal add to serve the other party noice. But an attempt to find the other party has to be dohe first