First, one should already be proficient as a perioperative nurse. Then you get further preparation in a formal program given by either a college or a university. There is a voluntary RNFA certification exam. But one doesn't have to be certified as an RNFA to practice. You'd be surprised how many nurses move into it or add it to their resumes so they are qualified to do one more thing.
Most nurses like to have as broad a base of skills as possible.
In the changing healthcare environment, the broader your base of skills, the more likely you'll have a job. If a nurse is qualified to first assist, then she or he can be a circulating nurse, first assistant, or scrub nurse. It depends on the practice setting how much she first assists. That person is responsible for collaborating with the operating surgeon to achieve the best surgical outcome for the patient. There are some specific behaviors that are listed in the official statement, like tissue handling and using instruments.