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Well, the specifics would depend on your home's design. Though new flat-panel televisions make this project less necessary for many people, those of us who find a flat panel out of our price range, this is a good way to conceal a TV without a bulky cabinet. The basic procedure would be something like this:
First, decide where you want the TV. Since TV's have quite a bit of depth, even the small ones, you will either have to 1) build a cabinet deep enough to completely absorb the depth of it, or 2) open up the wall behind the TV to allow for less cabinet depth. If you have a closet behind the proposed location of the TV, you could install the TV on a shelf within the closet, and put the mirror flat on the wall!
For the mirror, you can purchase (or order) a piece of one-way glass from any good glazier. It must be precisely sized, of course, so measure it well. If you wanted to be really fancy, you could install the one-way glass into an existing hanging mirror, contemporary or antique, rather than just mounting the mirror on the wall with wall-mounting mirror clips.
If you are using the mirror alone as a cabinet door, you may be able to purchase the proper mirror mounting hardware (hinges and handle) from the same glazier. If you have a wooden panel door already installed, you may be able to remove the panel and replace it with the mirror by carefully cutting out the inside of the door frame to release the panel. Then, use a small molding deftly nailed in place with small brads to hold the mirror in place. You could also glue the mirror in with construction adhesive or clear caulking.
Don't use any thinner than 1/4" thick glass for any mirror. For a wall mount, thinner glass can be used, but I personally think the heavier mirror is a better choice. Aside from these considerations, you will also have to supply electrical power and your cable or antenna wire to the rear of the TV.
It is important that the room/space behind the TV be either totally dark and/or enclosed. I have seen TV installations where the rear of the TV opens into an attic or storage space. If there is any light entering from the rear of the mirror, it may show through. Remember the movie "Absolute Power" with Clint Eastwood?