Remove the original vanity door by unscrewing the hinges from the door. Save the screws. Take the measurements of the original door and transfer them to the new door board. Mark the new measurements against the new door board.
Cut the new door board to match the measurements of the old door. Sand all cut edges of the new door board until they are smooth to the touch.
Apply a light coat of stain or paint to the door, using a brush and long, even strokes. Match the direction of the strokes with the direction of the wood grain. Allow the first coat to dry for four hours, then apply another coat in the same manner.
Apply a coat of sealant once the second coat of paint or stain has dried completely. Use the same method as before, and apply three coats of sealant, waiting for the prior coat to dry each time. Allow the door to dry overnight before proceeding.
Hold the new door board up to the vanity. Mark the location of the hinge holes on the interior of the door. Drill out these holes, being very careful not to drill through to the other side of the door. Set the original screws from the previous door within the hinges and into the holes in the door. Tighten these screws until the door is firmly attached to the hinges and the vanity.