We describe here lessons learned in designing an early algebra curriculum to measure early algebra's impact on children's algebra readiness for middle grades. The curriculum was developed to supplement regular mathematics instruction in Grades K-5. Lessons learned centered around the importance of several key factors, including using conceptual frameworks to design the components of our curriculum, treating early algebra content as a set of core algebraic thinking practices across several core content domains, and following particular stages in curriculum design research. We also learned the importance of recognizing and addressing "gaps" in the assessment tools available for measuring growth in students' learning, in the empirical research base for early algebra, and in support for teachers around professional learning in early algebra. We found curriculum design to be a complex and expensive process that required careful pacing and deliberation to address these gaps. [This chapter was published in: "Lessons learned from research on mathematics curriculum," edited by D. R. Thompson et al., Information Age Press, 2023.]