Sand one 3-by-1-by-24-inch board and two 14-by-1-by-24-inch boards with fine-grit sandpaper, smoothing all sides and edges. Apply a coat of wood stain to the front of each piece and wipe off the excess with a rag after 60 seconds. Once the stain dries, turn the wood over and apply stain to the back.
Lay two of the 14-by-1-by-24-inch boards on top of each other. Measure up 3 inches from the 14-inch side and mark the spot. Place a circular saw on the top edge of the 24-inch side. Carefully cut through both layers of wood, creating a 45-degree angle from the top to the 3-inch mark.
Set the third 14-by-1-by-24-inch board upright. The 24-inch side is the length, the 14-inch side the height and 1-inch side the width. Apply wood glue to the ends of the board. Line the two boards you cut in the last step against the wood glue, creating a three-sided frame. Apply wood glue to the 3-inch ends and place the 3-by-1-by-24-inch board on the front, creating the desk base.
Apply wood glue to the edges of a board measuring 24 inches by 14 inches by 1 inch and set the frame from the last step on top. Clamp the wood boards together and let dry overnight. Remove the clamps and turn the box upside down. Sink 1 1/2-inch wood screws through the bottom boards and into the sides. Attach the screws every 2 to 3 inches.
Turn the base upside down and make four marks near each corner, marking the location of the legs. Hold a 28-inch desk leg flat against one mark and screw 1 1/2-inch wood screws through the leg and into the wood. The leg should have three to four holes on plate surrounding the leg. Repeat with the remaining three legs.
Attach two hinges to the 24-inch side of the second board measuring 24 inches by 14 inches by 1 inch. Screw one hinge 2 inches from each end. Open the hinges and lay the board flat on the box. Lift up the top board and screw the hinges to the inside of the base.