Marine ecosystems are increasingly threatened by the cumulative impacts of multiple stressors, including climate change, pollution, habitat degradation, and resource overextraction. These cumulative impacts are expected to intensify as Blue Economy development continues to expand. Research has advanced cumulative impact assessment methods, but a gap remains between knowledge and its application in marine spatial planning. Complexity of cumulative impact assessments often leaves industries uncertain as to what cumulative impacts are, how they can be incorporated into decisions for planning and management, and what tools are available to account for them. Global cumulative impact assessment guidelines often contain inconsistencies in terminology, definitions, methods, and scope. Here, we provide a primer for development proponents and managers, defining cumulative impacts across scales, comparing qualitative and quantitative methods, and outlining a workflow for incorporating cumulative impacts into marine spatial planning. We provide the essential information for decision makers to translate best-available cumulative impact research into practice and bridge the research-implementation gap in cumulative impact assessments.