Scaling up breastfeeding programmes has not been highly prioritized despite overwhelming evidence that breastfeeding benefits the health of mothers and children. Lack of evidence-based tools for scaling up may deter countries from prioritizing breastfeeding. To fill this gap, Becoming Breastfeeding Friendly (BBF) was developed to guide countries in effectively scaling up programmes to protect, promote, and support breastfeeding. BBF includes an evidence-based toolbox that consists of a BBF Index, case studies, and a 5-meeting process. These three interrelated components enable countries to assess their breastfeeding scaling up environment, identify gaps, propose policy recommendations, develop a scaling up plan, and track progress. The toolbox was developed based on current evidence and expert guidance from a Technical Advisory Group, which was composed of global breastfeeding and metric experts with experience in the scaling up of health and nutrition programmes in low-, middle-, and high-income countries. The BBF toolbox required a step-by-step iterative approach to describe and systematize each component, thus an operational manual was developed. The BBF toolbox and BBF operational manual underwent intensive pretesting in two countries, Ghana and Mexico, resulting in the modification of each component plus the operational manual. Pretesting continues in six additional countries demonstrating that BBF is a robust and dynamic multi-sectoral process that, with relatively minor adaptations, can be successfully implemented in countries across world regions.