Overlay Scene

4.  Add a camera to the Overlay Scene
  •  3D View menu >> Add >> Camera
  • The overlay scene must have camera.

  • The position and rotation of the camera doesn't matter.

5.  Switch to Camera View
  • Select camera

  • 3D View menu >> View >> Cameras >> Set Active Object as Camera

  • The active camera view is displayed as the overlay.
6. Add the object in front of the camera.
  • 3D View >> Add >> Mesh >> Plane

  • Position/Resize Plane

  • For this tutorial, I'm adding a simple Plane.  The finished blend has a weapon reticle.