Gather the measurements of the relevant door frame. Use a pencil to mark the measurements onto a piece of wood.
Cut the wood to match the measurements, using a circular saw.
Adjust the thickness of the door to your liking by using a wood planer.
Mark the piece of wood at 36 inches high and 2 3/4 inches away from the right edge, using a pencil. Drill a hole saw bit over the mark to create the hole for the doorknob.
Switch the drill so that is loaded with a paddle bit, and create the door's latch opening on the right side of the door, at the same height as the doorknob hole.
Use medium-grit sandpaper to smooth the surface area of the door. Wipe away any sawdust with a clean rag.
Cover the door with a thin layer of wood stain in the shade of your choice, according to the directions on the specific stain. Allow the door to dry completely before installing the doorknob and hinges.