The purpose of this study was to elicit design guidelines for a teacher tool to support students' diverse needs by facilitating differentiated instructions (DIs). The study used a framework based on activity theory and principles from universal design for learning. As for the research methods, design-based research methods were adopted, and as the first step, this study selected and interviewed four teachers and five community members. As a result, it identified facilitating and conflicting factors in practicing DI and analyzing the activity system in teaching for DI. From this analysis, specific user requirements were identified as blueprints for the design of new tools as mediating strategies. Furthermore, the findings helped establish five design guidelines for teacher tools to encourage DI practice. This study has implications for a teacher application as a mediating tool, which will facilitate DI practice by developing an understanding of teachers' needs and the challenges they face in DI activities. It also presents a methodology for eliciting users' requirements as the first step of design-based research to leads to innovations embodied in specific theoretical claims.