How did the slave girl at Philippi earn money for her masters?

Answer: By fortune-telling

She had a spirit of divination and earned much by fortune-telling (KJV: soothsaying) until Paul cast the spirit out.

“One day as we were going to the place of prayer, we were met by a slave girl with a spirit of divination, who earned a large income for her masters by fortune-telling.”— Acts 16:16 (BSB)