You need 1 picture frame, with glass and cardboard backing.
Glossy Black enamel spray paint
Carefully remove the glass. I cut myself, and I was careful. The edges are going to be sharp. I considered it an unintended blood sacrifice.
Clean and dry the glass thoroughly.
Lay the glass flat on newspapers.
Spray thin and even coats. Allow time to dry between coats. Do not let it run. Do not touch the painted area to test for dryness. In fact, don’t touch it at all until you are done.
You may notice the paint not coating the glass evenly. You may see lighter colored spots. Don’t worry about it. That will be fixed.
Now paint the cardboard backing. This will reflect back any light that makes it thru the mirror.
Wait until everything is dry and reassemble the mirror, with the painted surface of the glass facing the rear.
How to use the mirror
That will have to wait, until I have some actual success with it. When I do, I will write about it.
In the meantime, there is THIS SITE that may give you some ideas.