In this paper, we introduce a novel method for the synthesis of qualitative data and co-production in the development of evidence-based guidelines. The call for evidence-based practice in healthcare settings has been dominated by a focus on patient groups, overlooking the need for robust guidelines to inform the delivery of support or developmental interventions for staff members. We propose an eight-step method that brings together primary and secondary qualitative data with co-produced data. Data is synthesised at two of the eight stages. This method generates robust findings and recommendations which are well suited to informing written guidelines. We present our experience of implementing this method in the development of postvention guidelines for the support of National Health Service (NHS) staff following the death by suicide of a colleague. This worked example illustrates the application of the method to the generation of evidence-based, co-produced practice guidelines. We discuss the application of qualitative data and co-production in the development of fit for purpose guidelines, and the lack of transparent reporting of methods in existing guidelines. We recommend that guidelines should be underpinned by empirical evidence and developed in consultation with stakeholders and end-users, including those who will implement and those who will receive treatment or intervention. For transparency and to inform end-users, we conclude that written guidelines should always detail the methods employed in their production.