Secondary mathematics teachers' use of formative assessments have shown promise for developing models of their students' mathematical thinking and informing their instruction. While the complexities of secondary mathematics teachers' formative assessment practices are often captured in qualitative research, there is a critical need for synthesized recommendations to connect formative assessment theory to practice. In a meta-aggregation synthesis from 11 qualitative manuscripts, we explored in-service teachers' formative assessment practices in US secondary mathematics classrooms. Our synthesis led to nine recommendations for in-service secondary mathematics teachers throughout three phases of their instruction: (a) prior to gathering evidence of student thinking; (b) while gathering, supporting, and responding to student thinking; and (c) reflecting on formative assessment practices. We close with connections to equitable teaching practices in secondary mathematics classrooms.