To unwrap a 3D model for opacity (preparing it for an opacity map) without Substance, use tools like Blender, UVLayout, or Modo—focus on marking clean seams and non-overlapping UV islands.
Mark seams along less visible edges (e.g., creases or back faces) to guide unwrapping; use your tool’s unwrap function (like Blender’s Smart UV Project) and adjust islands to avoid stretching—this ensures your opacity map applies evenly.
For beginners, Blender’s free UV tools work well: mark seams with Ctrl+E, unwrap via the U key, export UVs to paint an opacity map in GIMP or Photoshop, then test the map in your 3D viewport to fix distortion.