Whether you're making a Goetia-style circle-in-a-triangle design, or some other design, you'll need at least two things: a piece of glass the size and shape you want for your reflective surface and some flat black spray paint. Be sure your work area is well ventilated.
Outdoors is probably best if possible, but not if it's dusty and windy. Lay the glass (removed from any frame you plan to use) on a several sheets of newspaper or a drop cloth, with the side you intend to use as the viewing surface faced down and back side-up, clean, and dry. Shake the spray paint per directions on the can. Then spray the paint with a sweeping motion from side to side across the surface of the glass. Start at the top and sweep across, move down a little and sweep across again, etc., till the entire surface is coated with paint. Allow to dry for at least two hours, and then apply another coat. Some people apply as many as 13 coats of paint, but 2-3 is usually plenty. Leave it alone for at least 24 hours. Pick it up and carefully clean off the front (viewing) surface. The glass is now ready to be framed or mounted.
Freestyle
A common method of making a magick mirror is to buy a non-reflective picture frame and use the glass that comes with it. The mirror surface can be any shape but oval or circular is most common. A square frame with an oval or circular matte is not uncommon.
Goetia-style ala Runyon/Kraig
The method developed by Carroll "Poke" Runyon (also published by Donald Michael Kraig) entails cutting a triangle out of wood and painting it with names and symbols (usually from Goetia), and then mounting a circular black mirror on it.
The glass may be mounted a number of ways. L-shaped mirror mounts will work well, but talk to people at the hardware store about the smallest size that will securely hold the weight of your mirror. The triangle can be set on an easel on a slight angle. Alternatively, you could install a prop on the back to make a table-top model (look at any table-top picture frame to see how that's done).
Size should be determined by how much space you have, and your desires (if you prefer to stand or sit, etc.), however, you want to avoid reflecting the candle flames in the mirror surface, which can become a problem if the mirror is too large.
Alternatives
As an alternative to glass, you could use any dark, reflective surface, including polished black marble or onyx, a dark ceramic dinner plate, etc., which could be set on a bookstand on a table. The general idea is to avoid the sharp reflection of a regular mirror. A soft, somewhat obscured reflection is the goal