@Mike53

Any chance someone could explain this in a step by step manner, i keep getting something that looks like frosted glass.
Appreciate any help

You must follow the steps cited by Puybaret.
Download the rectangular mirror as a template.

To sum up, you just have to add the prefix [font=courier new]sweethome3d_window_mirror[/font] to the part of the object that you want to turn into a mirror in its .obj file.