Before building the new door, it is important to remove the old door and use it as a template to get the correct size. Begin by unscrewing the hinges attached to the door. It is not necessary to remove the part of the hinge attached to the inside of the vanity cabinet since those will be reused.
Once the door is off, grab a tape measure and determine the length and height of the old door.
Cut the new door structure out of 3/8-inch MDF, a great material that typically costs less than natural wood and has a tendency not to split like grained wood. Measure the door panel dimensions on the MDF and cut the board appropriately with a chop saw. Be sure to wear safety goggles when using the saw.
Use the chop saw to cut down rips of 2-inches for the raised rails (one on the top and one on the bottom) and stiles (one on the left side and one on the right). Measure the rails to the length of the board and keep in mind that the stiles will fit inside the rails around the perimeter.
Once the four pieces are cut, place a moderate amount of wood glue on the back of one of the rails and fit it on to the top of the board face. Next, use the brad nailer and tack a few nails into the rail to secure it into place. Don't worry if there's a slight overlay or profile showing because the edges will be sanded down once everything's in place. Repeat the gluing and nailing steps for the two stiles.
Glue and nail the bottom rail on to the board, making sure it touches the stiles as closely as possible for a tight, seamless fit.
Pour some paint of your choice into a paint tray and grab a paintbrush and roller. It will be easier to paint the space within the frame with the paintbrush to make sure to cover all the edges. For a unique look, rolling paint on to the rails and stiles give it a contemporary, textured feel. Be sure to paint the underside of the door once the paint dries so that no moisture seeps in and warps the wood.