Arrange long boards that are 1 1/2 inches thick and slightly longer than the door frame so they are parallel to each other and resting on their largest surfaces with their long, thin surfaces touching. Set three boards that are 1/2 inch thick and slightly shorter than the door frame's width horizontally across the long boards. Arrange the shorter boards so that they are evenly spaced with their largest surfaces touching the long boards.
Drill two holes through each short board and into each long board that it touches. Set 1-inch long wood screws in each hole, and fasten them securely until all the boards are installed tightly. This task creates the initial door.
Measure the height of the door frame. Use a saw to cut the length of the door so it is just 1/2 inch smaller than the door frame's height. Sand the edges of the door and all visible surfaces.
Apply clear-drying caulk in all large gaps and knotholes in the door's wood. Allow four hours for the caulk to dry.
Apply a coat of paint or stain, if desired, by brushing it on in smooth, even strokes that follow the wood's grain. Allow two hours for the first coat to dry, and then reapply additional coats with the same method until the door is the desired color. Repeat the procedure with three coats of sealant.
Ask an assistant hold the door in the door frame so that it is positioned correctly. Hold three evenly spaced door hinges against the door and the door frame, and mark the position of the hinges' screw holes. Drill each hole, and attach the hinges to the door and door frame with 1-inch long screws. Test whether or not the door opens smoothly in the door frame. Adjust the hinges or sand the door's edges to ensure ease of operation.
Position a doorknob on the door, and drill a hole for the doorknob bolt to pass through. Drill holes along the edge of the door and through into the doorknob to provide space for the latch mechanism. Fit the latch mechanism and the doorknob together in the holes, and tighten them with the doorknob's included installation equipment.
Close the door, and mark where the door latch hits the door frame. Drill a rectangle on the frame for the frame plate to be installed. Position the frame plate over the rectangle, and install it with the screws that were included with the doorknob. Test the door to ensure it latches, and adjust it as needed.