This paper aims to build a generic roadmap for implementing Lean in different domains in order to improve the business processes. The customization of this roadmap makes it possible to adapt the steps to be followed for Lean implementation, the process manager’s requirements and available resources. An application has been developed in order to facilitate the customization of this roadmap. An analysis of methods from a literature review for Lean implementation was conducted in order to identify the steps to be included in the roadmap. A rigorous approach based on set theory, was therefore proposed and followed to build the roadmap. This roadmap is defined as a set of phases associated with principles, and each phase is defined as a set of steps associated with tools. This roadmap shows to the process manager, the objectives to aim for, the Lean principles to satisfy, and the Lean tools to use.