Formative assessment can facilitate teachers' abilities to elicit and notice the disciplinary substance of students' thinking and to respond based on this. Following a design-based process, we developed principled practical knowledge to create resources that might guide experienced teachers in examining their formative assessment practice and provide researchers with tools to study formative assessment enactment. Starting with the outcomes of the first cycle to generate a generalizable explanatory model, we describe the process of our second cycle, which resulted in a practical guide and research tool. The complexity, challenges, and major learnings of the design-based process are highlighted. Implications for teachers' professional development are discussed. [For the complete volume, "Bridging Research and Practice in Science Education: Selected Papers from the ESERA 2017 Conference. Contributions from Science Education Research. Volume 6," see ED615249.]