I actually made mine a couple of years ago. I went to the craft store and bought an antique-looking picture frame and some flat black paint, and took it home. After laying out some newspaper, I removed the back from the frame and slid out the glass, cleaned it, and then put it on the paper. I sprayed the back of the glass with a coat of the black paint, let it dry, and then applied another one to make sure that it was evenly coated and completely black. After it dried, I put the glass back into the frame (with the unpainted side facing OUT) and then put the back back on. After performing an empowering rite, the mirror was ready for use.
I recommend using it in a very dark room, with only one or two candles going, but be sure to place them where you don't see the reflection in the glass, because you want to have a perfectly smooth reflection. You can also burn a divination incense, consisting of Mugwort, Wormwood, or others. Place the mirror on your worktable or similar, standing on edge, and sit about 4-6 feet away. Again, be sure that nothing is actually reflected in the glass; you want a perfectly black image to gaze into.
My own experiences with the mirror have been promising, but to be honest I haven't spend a huge amount of time with it. The first time I used it, I did began to see images, but felt I might not be ready for anything else so I stopped my experiment. In my case, after staring at the glass for about 5-10 minutes, I started to see what looked like a glowing mist forming. Shortly after, I began to see indistinct images (random shapes and such) swirling about, which then began to feel three dimensional and actually looked like they were coming OUT of the mirror, flying at me. It was definitely a witchy, magical experience but not something I would recommend if you are of a nervous or unsure disposition. Out of all the "gazing" methods of divination I have tried, the Dark Mirror had the best and most rapid results, quicker and better than a crystal ball or similar.
I do hope this has been instructive. Here's an image of my Dark Mirror at work, so hopefully you can get an idea of the technique I use.

