SPACES (Supporting Physical Activity through Co-production in people with Severe Mental Illness) is a study which aims to develop an intervention to increase physical activity created with and for people with severe mental ill health (SMI), their carers and professionals involved in physical activity and/or severe mental ill health. People with SMI are less physically active than the general population and have an increased likelihood of experiencing long-term physical health conditions such as cardiovascular and respiratory diseases, diabetes and obesity. The SPACES team employed a comprehensive process of Patient and Public Involvement and Engagement (PPIE) work embedded within a co-production strategy. Researchers worked together from the point of inception with people with lived experience, two of whom became co-applicants, to design and carry out the intervention development stage of the study. This included PPIE work and an iterative process of focus groups and interviews with various stakeholders and a consensus group made up of multiple stakeholders with lived, caring and professional experience. Here, we describe the co-production model we used, the benefits, challenges, achievements and areas for learning and improvement. We offer co-production principles and practical strategy, which we hope will be used, modified, personalised and built on by others. We also offer the idea that laying out the co-production strategy to be employed prior to a study commencing and then comparing how that strategy was or was not met could be a step towards creating more accountability and academic rigour in co-production.