One of two ways:
1. Anytime you write to a statistic, you're updating the leaderboard for that Statistic ID. So, if you write to a statistic ID that's never been written to before in your game, that'll create a new leaderboard for that ID, with a default configuration (no timed resets, "last" aggregation).
2. By using the Admin/CreatePlayerStatisticDefinition API call.
1 Person is following this question.