Remove the original door, if applicable, by loosing the screws from the hinges on the door. Set the screws in a safe place and detach the door board. Set the old door board on top of a sheet of plywood for the new board.
Trace the outline of the old door onto the new plywood door board, or take the measurements of the opening using a tape measure and transfer those measurements to the new plywood door board. Make an outline of the board using a T-square or yardstick, and cut out the new door using a saw.
Sand the edges of the door board thoroughly. Wipe the entire surface down to ensure that no sawdust remains. Apply a thin coat of paint or stain to the entire board. Work in smooth, even strokes that follow the direction of the wood grain. Allow the first coat to dry for four hours, and then apply a second coat.
Continue the process until the color is to your specifications, and then apply a coat of sealant using the same procedure. Allow the sealant to dry for four hours, then repeat to add three coats. Let the door dry overnight before installing.
Position the door against the old hinges. Mark the location of the screw holes on the inside of the door. Remove the door and drill into each marking using a drill bit that is smaller slightly in diameter than the hinge screws you removed earlier. Set the door back against the hinges and drill the hinge screws through the hinges and into the door board.
Position a knob on the outside of the door and mark the location. Drill a hole with the same diameter as the knob's attachment bolt and thread the bolt through the hole. Tighten the knob's nut over the bolt firmly.
Open the door and position a magnetic latch on the inside of the door, along the middle of the side board that doesn't have the hinges. Mark the positions of the screw attachments for the latch piece on the side board and on the rear of the door. Drill holes at these markings, then screw the latch pieces in place with 1/2-inch wood screws.