Remove the pins in the door's hinges using a screwdriver and hammer. Remove the door from the hinges and place it on two sawhorses, making sure the door is stable on the supports.
Use a screwdriver to remove the screws that hold the door's handle in place, then pull the handle apart and set it aside. Remove any other pieces of hardware that sit on either side of the door.
Mark exactly where you want the molding to be on the door using a measuring tape, a pencil and a square. Step back and examine the markings on the door to make sure you do not want to make any changes before continuing.
Cut lengths of molding by placing them in a miter box and using a wood saw to make the cuts. Make sure you use the same degree of mitering for each of the pieces of molding by using only one of the slots in the mitering box to guide the saw.
Sand the pieces of molding lightly where you made the cuts. Paint the molding the color you desire and allow it to completely dry.
Apply wood glue to the back of each piece of molding and place the molding onto the door, using the pencil marks as guides. Push brads into several places on each piece of molding, attaching it to the door, using a brad pusher. Allow the glue to dry, then reinstall the door handle. Place the door back on its hinges and use the hammer to drive the pins back into the hinges.