The black mirror is used very much like a crystal ball.
You can purchase, or make your own if you prefer!
Black paint on the back of a piece of glass is all this takes.
Use an old photo frame, paint the back of the glass and return to the frame, etc.
Gaze into the mirror, trying to find its center, and keeping your sight clear.
When you begin to see grey or white clouds, whisper your query, and see what the response is.
You can use the mirror to cast The Companion Quest but only do this if you truly want to invite a spirit or entity to come close to you.
Some have done this with success and are quite pleased with the result, but it’s a matter of how comfortable you feel about having an “invisible” presence that makes itself known, to you.
If talking to spirits is a matter of course for you, this may be nothing new or different, and you may even find the presence reassuring.
But as with anything else, be sure this is what you want (with your mind and with your heart) before you proceed.