Step1
Make these cuts:
Plywood:
1 piece @ 24 inches x 48 inches
1 piece @ 4 inches x 48 inches
1 piece @ 4 inches x 46 inches mitered @ 45 degrees on one side of both ends
2 pieces @ 4 inches x 23 inches mitered @ 45 degrees on one side of one end.
2x4s:
2 pieces @ 20 inches x 2 inches x 1 1/2 inches
2 pieces @ 15 inches x 2 inches x 1 1/2 inches
2 pieces @ 35 inches x 2 inches (to be mitered @ 45 degrees later)
Step3
Build the supports. Attach a 20 inch 2x4 piece to a 15 inch 2x4 piece at a 90 degree angle making an "L" that is 20 inches x 16 1/2 inches.
Make sure the pieces are square to each other. Place a 35 inch 2x4 piece on top of the others making a triangle. Mark the piece for length so that it will fit snugly inside. Miter the ends of the 35 inch piece and attach. Repeat for the other brace.
Step4
Build the top. Attach the mitered plywood pieces together forming a 46 inch x 23 inch x 4 inch squared off "U". Attach the 24 inch x 48 inch piece to the top of the "U" so that it hangs 1 inch over each side, 1 inch over the front and is flush with the back.
Step5
Fill in all holes with wood filler and spot sand smooth. Edgeband the exposed edges of the plywood.
Step6
Paint or stain to your liking.
Step7
Using pocket screws, attach the 4 inch x 48 inch piece to the wall 29 inches off the floor. Make sure the piece is level. If you center the piece on a stud, the two braces should also be able to attach into studs, though sometimes stud placement varies. Improvise if necessary.
Step8
Attach the heavy duty hinges to the braces one above the other. Attach the other side of the hinges to the wall (studs) so that the braces touch the bottom of the 4 inch x 48 inche plywood piece. When the hinges are closed, the triangles will face in toward each other. When open they will stand @ 90 degree angles from the wall.
Step9
Attach the 2 piano hinges to the back of the top piece. Attach the other half of the piano hinges to the 4 inch x 48 inch piece of plywood. Make certain the pieces are flush on both ends.
Authority 
