Use a measuring tape to determine how tall, wide and deep you need your fireplace to be for your set.
Cut four of the correct width of board -- for example, two-by-eight boards for an 8-inch deep fireplace or plywood if you need a much deeper fireplace -- to your needed width and height for the project.
Screw all four boards together to create a box.
Screw the bottom board to a piece of plywood, if your fireplace needs to stand alone, to keep the unit from falling over.
Cut a rectangular piece of plywood just large enough to cover the back of your fireplace.
Cut another piece of plywood into the shape and detail of your mantel. You can keep the front square, or use a jigsaw to create a more unique shape. Use wood glue and small screws or nails to secure the front to the fireplace to prevent splitting the wood.
Drill the necessary holes for your fireplace lighting in the back of the fireplace if you plan to use a DMX-controlled fixture for a more realistic look, or plan to use an electric fireplace insert.
Paint your fireplace front and box to match the genre and time period of your play. You can also employ stack stone or fake brick panels to create an even more dimensional and realistic look.
Drill a handle into each side of the fireplace so that they are hidden behind the front piece. This will allow for easy moving of the fireplace set in and out for different scenes. You do not need handles if you plan to make the fireplace part of a larger moving or stationary set piece.