To be self-confident about a particular ability or action means to think of oneself as competent in that area (e.g. judgment, ability, power, skill in taking a type of action). If self-confidence is a sense of competence and competence is developed through practice, then we can develop self-confidence by practicing the ability or skill we wish to be confident about.
If this is so, then we can formulate a practical maxim for developing self-confidence:
To develop self-confidence, practice doing the thing you want to be confident about until you think of yourself as competent in doing it.