In this study, we present a conceptual tool for guiding teachers' principled pedagogical actions toward equitable instruction, referred to as the "Transforming Science Learning" (TSL) framework. The TSL framework was developed to address the challenges of enacting an ideological commitment in local contexts--promoting equity and justice through culturally relevant pedagogy (CRP) in K-12 science classrooms. TSL consists of five design principles that articulate the goals (the "why" of practice), instead of prescribing pedagogical activities (the "what" of practice). The five principles are: a) make it matter, b) support sense-making, c) attend to race, language, and identities, d) build a welcoming community, and e) disrupt power hierarchies. We use a co-designed high school physics unit enacted by one teacher, Ms. Davis (pseudonym), as an illustrative case to contextualize our overall data analysis in three physics classrooms. We examine what teachers did, guided by each design principle, including when they planned lessons, interacted with students during instruction, and assessed student learning. We discuss the affordances of a well-designed conceptual tool in addressing the problem of enactment by productively mediating co-design toward transformative and consequential learning.