The 2023 has a broad scope aligned with WHO's responsibility to develop a comprehensive guideline as part of the Global Action Plan on Child Wasting. A WHO technical team consisting of the WHO Steering Committee for this guideline, supported by a consultant and two independent methodologists, coordinated guideline development according to WHO's standards and methods. The guideline contains 21 recommendations and 12 good practice statements on prevention and management across four focus areas. As can be expected from a broad guideline covering many new areas, we faced complexities. As a technical team involved throughout the 3-year process, we reflected on the challenges and our experiences in addressing these. In this paper, we summarise selected elements of processes and methods and our key reflections and learnings relevant to three challenging areas that may be useful to other guideline developers and groups addressing complex needs across contexts. Key reflections included (1) implementing comprehensive and systematic prioritisation was time well spent, allowing the most pressing guidance gaps to be addressed first, while also identifying possible future gaps; (2) evidence syntheses and evidence-to-decision approaches for broad and composite guideline questions required careful planning and coordination, which helped streamline Guideline Development Group discussions and consensus during decision-making and (3) putting steps in place aimed at ensuring consistency and rigour across multiple questions, reviews and meetings improved transparent and systematic guideline development processes.