Most conservation research aims to inform management of environmental challenges, but scientific evidence is used inconsistently in environmental programmes and practice. We used semi-structured retrospective interviews to ask 12 environmental scientists and 14 practitioners (land managers, park rangers, project managers and planners from natural resource management agencies) about factors that facilitated and hindered the use of scientific input during 15 environmental projects. We used the common factors from interviews to develop a process model describing how scientific input informs programmes and practice. The model emphasised the social dimensions of environmental projects which are often overlooked when these projects are planned, managed and evaluated. It highlighted the pivotal role of relationships in achieving outcomes which include creating practical, useful products and tools, and robust, credible and trusted evidence. By clarifying the process of how scientific knowledge informs environmental programmes and practice, the model enabled us to provide guidance about how to undertake transdisciplinary work and suggest indicators to track progress. Although derived from environmental projects, the guidance is likely to apply to other fields, particularly where different disciplines work together.