Writing in mathematics provides students with the opportunity to think critically about and reflect on their experiences while solving problems. While many studies have documented the benefit of writing in math, it is not clear which instructional methods should be used to help students learn how to use writing to support learning. In this study, we take a constructivist approach to building students understanding of effective writing by developing a series of active, student-centered lessons. The findings indicate that students wrote more effectively after the instructional sequence; in particular, they were better able to explain their reasoning and to make connections between abstract mathematics and the context of the problem. These results provide a pathway for improving both students' conceptual understanding, and their performance on open-ended state assessment items.