The best way to flat shade an environment 3D model is to use per-face normals (instead of smooth normals) and apply solid-color, unlit materials—this ensures each polygon (e.g., walls, floors, terrain) has sharp, distinct edges for a stylized or minimalistic look.
In Blender: Go to Edit Mode > Normals > Set to Face to enable per-face normals. Then use a basic material with no specular/roughness maps—stick to solid colors.
In Unity/Unreal: Disable normal smoothing on your model and use an unlit shader (like "Unlit Color" in Unity or "Unlit Material" in Unreal) to avoid gradient lighting.
This method works across tools and keeps the flat-shaded style consistent for environment assets.