From http://www.llewellynencyclopedia.com/article/5154

Making and using a magic mirror is an ancient tradition. There are many different variations on how to do this that have developed over the centuries, but the basics are very simple, especially since it is now possible to find the necessary curved piece of glass.

1) Obtain a clear piece of glass that is curved like a bowl. These can be obtained from watchmakers or clockmakers. Alternatively, they are sometimes found in hobby shops. A hand-held one can be as small as three or four inches in diameter. Larger ones can be used by placing them on stands or keeping them in boxes. Some people like to line the box in black velvet.
2) Wash the glass thoroughly and dry it with a lint-free cloth. Some people like to use special herbal washes, including such items as eyebright.
3) If you were to use the glass to hold water, the part that holds the water is the inside while the part that would rest on a table is the outside. To make a magic mirror, you paint the outside flat black. Check with the store where you buy the paint, as some paints do not adhere well to glass. In the past, I’ve used paints designed for blackboards to great effect. It is better to apply several thin coats (being sure to let them dry thoroughly between coats) than one heavy coat, as the result may not be smooth. Do as many coats as necessary so that you cannot see through to the other side when you look at a light through the glass. Some people apply a mystically or magically significant number of coats of paint. When you look into the bowl, you should see no streaks or runs in the paint. If you do, remove the paint and start again at step two above.

1) Begin by doing any cleansing or banishing rituals you wish to use.
2) If you work with any deities, call on them for support. If you are going to do a divination for a specific purpose, clearly identify what that purpose is. You might even want to write this down on a piece of paper.
3) Dim the lights and darken the room, save for one small light. Many people prefer a candle. Hold the mirror so you are looking into the inside of the glass, similar to looking into a bowl. That is, the painted part is on the far side of the glass from where you are looking. If you used flat black paint, the glass will make it look shiny. You should not be looking at the side with the flat black paint on it. If it is too big to hold, set its holder with the mirror on it (or in it) on a table in front of you.
4) Move the light so you cannot see its reflection directly in the mirror, but make sure that you can see the mirror.
5) Focus on the purpose of the divination and look into the mirror—not at it, but literally into it or through it.
6) Relax. Breathe. Allow yourself to be open to anything you might see, feel, hear, smell, taste, or sense. Nothing may happen the first few times you try this or you may be flooded with images. Whether it occurs right away or after a few experiments, it will come! Be open to anything. You don’t need to force it or try. Just relax and allow yourself to be shown what you need to see. Eventually, it will come.
7) Observe and remember what happens.
8) Finish with thanks to any you have called and dismiss them. Cleanse and/or banish the area. It is important to immediately write down what you experienced, as divinations of this kind can fade rapidly from memory, just as even the most vivid dreams may quickly fade from memory when you awaken in the morning.
9) Sometimes, divination with a magic mirror opens the divination rather than completes it. You may have prophetic dreams or synchronistic experiences as a result of the work begun with the magic mirror. Don’t ignore this new information.