Measure the length and width of your doorway. Subtract 1/4 inch from the width and 1/2 inch from the length to allow the door to open correctly. Cut the 1-by-6 wood plank into two pieces that measure the width of the doorway. These will be the top and bottom of the door.
Lay the two cut pieces of wood on the ground, spaced to fit the measurement of the doorway. Measure the distance between the top and bottom planks. Use this length to cut the three 1-by-12 planks to fit between the top and bottom pieces.
Place the cut 1-by-12 planks side by side between the top and bottom 1-by-6s. If the 1-by-12s are wider than the top and bottom pieces, you'll need to reduce the width of each 1-by-12 equally.
Divide the 1-by-4 into three pieces to form the diagonal frame board that will go on the inside of the door. The exterior of the door will have all vertical boards. Cut two of the pieces to be slightly less than the width of the door, about 2 feet. The third piece will need to be long enough to connect opposite ends of the top and bottom of the door frame to form a Z.
Use wood adhesive to connect the vertical 1-by-12 planks side by side and to the top and bottom 1-by-6 planks. Once the wood adhesive dries, attach the frame boards. Lay the two 2-foot 1-by-4 planks parallel over the top and bottom seams. Place the long 1-by-4 plank diagonally to complete the Z form. Fasten the frame boards to the door with sheetrock screws.
Use a semi-gloss paint of your choice. If the wood you use is old, it will absorb the first coat and require a second coat for even coverage. Once the paint is dry, attach a door handle and hinges with the drill and screws.