Add one Occupation attribute, in the Facts tab of the Property Box, for each of the person's occupations (see How to Record Facts About a Person's Life).
It often happens that a person may have had multiple occupations, but one of them is much more important that others, and may even form a part of their identity. In that case, you may wish to mark the relevant attribute as Preferred. See How to Flag a Fact as Preferred, Private, Tentative or Rejected to learn how to do this.
See also How to View an Individual's Record in the Property Box.