The phrase "flesh out" is commonly used to mean adding more detail or substance to an idea or plan. There are a number of synonyms that can be used in place of "flesh out," including "elaborate," "expand," "develop," "enrich," "enhance," "elucidate," "specify," "clarify," and "delineate." All of these terms essentially mean to provide more information and depth to an idea or concept. When choosing which synonym to use, it's important to consider the specific context and intended meaning. However, no matter which synonym is used, the goal is always the same: to add more substance and richness to an idea or plan.