To mix roughness for a 3D model, blend multiple roughness maps or adjust values in a material editor to create varied surface textures—combining a base map (for overall texture, e.g., matte plastic) with detail maps (scratches, dirt) using modes like multiply/add in tools like Blender or Substance Painter, mimicking real materials (worn metal, scuffed wood) with both smooth and rough areas.
For beginners, start with simple layer blending in your material editor: test small adjustments and use reference photos of real objects to guide how light interacts with their roughness.