In this case study, we describe some of the key analytical stages and decision-making processes during a research project comprising secondary analyses of the U.K. Millennium Cohort Study. Our work examined relationships between maternal body mass index (BMI) and breastfeeding behaviors. We discuss the development of our research: from familiarization with and appraisal of the proposed dataset; to choosing and recoding variables; to honing our conceptual framework and fitting the data to the research questions; to analysis, reporting, and discussing findings. While describing key stages, we also emphasize the iterative, messy, and nonlinear realities of conducting a research project using secondary data. We discuss crucial points to be aware of in this type of work and lessons learned: such as the need to update and revise coding, analyses, and interpretations in light of discussion, reflection, and external feedback.