Automated writing evaluation (AWE) tools can facilitate teachers' analysis of and feedback on students' writing. However, increasing evidence indicates that writing instructors experience challenges in implementing AWE tools successfully. For this reason, our development of the Writing Analytics Tool (WAT) has employed a participatory approach that includes teachers throughout all stages of design. WAT aims to generate writing analytics that are both useful to teachers and actionable by students. In this report, we discuss our participatory design approach and resulting insights, including (a) teachers' expressed instructional needs and goals, and (b) design solutions that were conceptualized to address both factors. [This paper was published in: "Companion Proceedings 12th International Conference on Learning Analytics & Knowledge (LAK22)," 2022.]