Outline the width and height of your secret compartment or closet’s opening onto the wall. Draw straight vertical and horizontal lines for the opening using a straight edge. Score the lines with a utility knife.
Make a small hole in the wall with a hammer. Check the inside of the wall to make sure it is clear of any electrical wires, heating ducts or plumbing. Remove the rest of the wall piece to completely open the doorway.
Cut two pieces of 2-by-4-inch wood using the vertical measurements of the wall opening and one piece of 2-by-4-inch wood for the top of the opening if it is a door. Cut another piece of 2-by-4-inch wood for the bottom of the opening if it is a compartment and not a closet.
Recess these pieces into the walls to be flush with the cut opening and nail them to the existing wall studs for support.
Frame the opening with the door framing and hang the door with hinges screwed into the door framing. Attach the hinges so that the door swings outward and not into the closet or compartment space.
Box in the secret closet space on the other side of the wall. Frame the base square with 2-by-4-inch wood and make a top plate to screw into the ceiling. Cut vertical pieces of 2-by-4-inch wood for the side walls and nail them to the top and base plates.
Cover the interior and exterior walls of the secret compartment or closet with sheetrock. Cut the pieces of sheetrock for smaller areas of the wall using a straight edge to draw the lines and a utility knife to score the lines. Snap the pieces of sheetrock apart by bending them on the lines and then cutting through the sheetrock backing.
Screw the sheetrock to the 2-by-4-inch wood studs, spacing the screws 12 inches apart along each stud. Spread joint compound over each screw hole to conceal the screw heads and over each seam. Lay paper or joint tape over each seam, and smooth another layer of joint compound over the tape until it is seamlessly smooth.
Install shelves in your secret space. Measure the width of the closet and cut 1-by-12-inch shelf boards to that measurement.
Cut 1-by-2-inch wood strips--12 inches for the two sides and the width of the closet for the back--for each shelf. Nail or screw these to the closet’s walls, and place the shelf boards on top.
Finish the secret compartment or closet by screwing the door to the back of the bookcase, mirror or furniture. Attach small wheels to the bookcase or large piece of furniture that will conceal the closet’s door. As the furniture is pulled away from the wall, the secret compartment or closet door will open.