IntroductionCultural adaptations of healthcare interventions are widely advocated to improve effectiveness, acceptability, and equity for diverse populations. Despite the existence of several cultural adaptation frameworks, there remains a lack of practical, transferable guidance on how adaptations are undertaken in applied healthcare settings. Adaptation processes are frequently underreported, limiting transparency, reproducibility, and learning across studies, especially within a UK context. This paper addresses this gap by presenting a structured, step-by-step guide for culturally adapting healthcare interventions, using a healthcare toolkit, 'I Manage My Meds', as a case study for adaptation.MethodsA methodological case study approach was used to develop an eight-step, iterative guide for cultural adaptation. The guide integrates a phased structure model with the substantive domains of the Ecological Validity Model (EVM). Development was informed by collaboration with community stakeholders, patients, public contributors, translators, and researchers during the cultural adaptation of an existing healthcare intervention for older adults from a South Asian background in the UK. The guide was refined through repeated cycles of stakeholder engagement, pilot testing, and reflection, with adaptations systematically mapped to the eight EVM domains.ResultsThe resulting step-by-step guide provides practical direction on how to plan, implement, document, and refine cultural adaptations across a healthcare intervention. Key reflections from the guide are that cultural adaptations should be considered as cyclical processes rather than linear; deep-level adaptations often require reframing intervention assumptions; and sustained stakeholder collaboration is essential for maintaining intervention fidelity while improving cultural relevance. The guide is designed to be transferable across populations, settings, and intervention types.ConclusionThis paper contributes practical methodological guidance to an underdeveloped area of implementation research. By offering a transparent and replicable step-by-step guide, it supports researchers and practitioners to move beyond superficial adaptations and to more consistently document cultural adaptation processes. Wider use of this guide may improve the quality, equity, and reproducibility of culturally adapted healthcare interventions.Patient or public contributionThe guide was developed in collaboration with the Leeds Older People's Forum and community representatives from a South Asian background who formed a stakeholder group. Alongside our lay leader expert, a co-author on the paper, they contributed to reviewing intervention content, identifying culturally relevant adaptations, testing pilot materials, and refining the step-by-step guide through iterative feedback and collaboration.