Place the edge of the tape measure against the floor and extend it upwards along the length of the door jamb to the casing at the top of the door opening. Do this on the side of the bi-fold doors that remains fixed in the overhead and floor brackets, not the door end that slides open and shut.
Mark the measurement for the height of the door opening on a strip of molding, using a pencil. Cut the molding along the mark, using a handsaw.
Close the bi-fold doors fully. Position the molding on the door jamb, about 1/8 inch away from the edge of the bi-fold door.
Nail the molding in place on the door jamb with brad nails, using a hammer. Use between four and six brad nails, spaced along the molding, to secure it. Tap the nails lightly with the hammer to install them.