Public health emergencies demand rapid expert guidance, but divergent expert opinions and mixed messaging can erode public trust and undermine efforts to change health behaviours. This paper describes the development and testing of TRICE (Template for Rapid Iterative Consensus of Experts), a structured method for generating psychologically-informed expert guidance quickly, with built-in peer review and consensus processes. TRICE was developed and piloted by the British Psychological Society's COVID-19 Behavioural Science and Disease Prevention Taskforce, a group of 15 core members drawn from across health psychology, social psychology and public health. The paper reports how the method was used to produce 18 peer-reviewed guidance documents and four rapid systematic reviews during the COVID-19 pandemic, with a median of 18 experts involved per document and a median of seven drafts per output. The process documents show how expert consensus was iteratively built and how the method balances speed of production with scientific rigour.