For a few decades now, scholars have tried to make mathematics education more meaningful and motivating by using narratives. However, there still remains a gap between the theory of narratives and classroom practice. In this paper we provide two design heuristics for teachers by which to bridge this gap. We discuss Dietiker’s mathematical story framework, based on narratology, and present a design heuristic based on this framework by which to design mathematical stories. We extend this heuristic on the basis of the work of the Russian formalist Vladimir Propp, who analyzed Russian folktales. He found that 31 irreducible narrative elements, called functions or narratemes, can be distinguished. We exemplify our heuristics by means of two mathematical stories, one of which concerns Kepler’s conjecture. The value of this approach is that it provides mathematics teachers with a heuristic story framework that is suitable for designing or redesigning mathematical stories.