Aerostructures assembly (ASA) is a vital process in any aircraft production phase that integrates individual detail parts, sub-assemblies, major assemblies, components, and systems into a final deliverable, a completed aircraft structure fit for flight. ASA in an aircraft’s entire product life cycle represents more than half the cost and time that is a significant portion of the total aircraft production cost. ASA depends on highly skilled manual labor work across the global aerospace supply chain for various assembly processes and subprocesses required for assembling detail parts into sub-assemblies and components to achieve the design intent of the load-carrying aerostructure that is airworthy for the complete operational cycle till disposal of an aircraft. The assembly processes can significantly impact quality, safety, and reliability and can affect an aircraft structure’s performance and design intent. To mitigate the increase in defects due to non-standardization and to fulfill the need for robust process designs of the assembly processes, this technical paper proposes the enhancement of the 6M methodology as a Design for Assembly (DfA) tool for developing both detailed process designs and robust Compliance Assessment Guidelines (CAG) for evaluating the performance of these assembly processes. The 6M methodology is proposed as an Aerospace Design for Assembly (ADfA) tool using the Design For Six Sigma (DFSS) approach to Define, Measure, Analyze, Design, and Validate (DMADV) the requirements of the ASA process designs. The 6M enhanced framework is used to create as a process design tool, and the outputs of the research are converted in assessment guidelines, which were validated by a global aerospace accreditation body for its adoption in the aerostructures supply chain. This paper can also be used as a reference guide by ASA engineers to readily identify the factors that can impact any process, ensuring the highest quality and reliable aerostructures products to customers ensuring cost-effective On-Time Delivery (OTD).