Mirror scrying is an evolved form of water scrying. When it became possible to build mirrors they were regarded as being like water that was fixed into one place.
The early mirrors were made of polished copper, brass, marcasite, tin foil or mercury behind glass, polished silver and obsidian. All types of mirrors may be used for scrying and the size is not important.
Because mirrors are linked to the moon mirrors should be backed with silver. Try and use a round or oval mirror instead of a square mirror.
For the frame try and use a mirror that has a silver frame. Old mirrors also seem to work better than new mirrors.
Most seers prefer to use a black mirror. Because this is difficult to buy you may have to make one.
Just simply take out the glass and paint it black. You may have to give it a few coats of paint though. When you put it back in the frame make sure the glass part is to the front. The use of black mirrors may be traced back over the centuries. John Dee used a black mirror of obsidian.
When using the black mirror for scrying you do not want to see your reflexion. The best is to leave the mirror on a table and look at it from an angle.
Look into the depths of the mirror as though you were looking into a bowl of water. At first it may appear grey than colours will come and go.
With time and practise you will be able to see scried images like still photographs or moving film images. Spirits may sometimes look at the scryer, talk to the scryer or even touch the scryer.
The visions may even exist outside the mirror and surround the scryer on all sides.