1. Measure the existing door threshold for length and width.
2. Purchase a metal sill of the same size and shape.
3. Use a variable speed drill fitted with a screwdriver bit to remove the screws anchoring the threshold into place.
4. Use a utility knife to cut through any caulk on the front and back edges of the sill.
5. Place the end of a pry bar beneath the threshold and pull up on the bar to pull the old threshold away from the door frame.
6. Lay the new threshold out on a flat surface and place the old one on top of it.
7. Use a pencil to trace the shape of the ends of the old threshold onto the new one.
8. Remove the old threshold and screw the new threshold down onto a work table to keep it from moving.
9. Use a jigsaw to cut out the shape of the door jambs from the ends of the new threshold.
10. Test the fit of the new threshold beneath the door and make sure the door closes over it.
11. Screw the new threshold into place.
12. Run a bead of waterproof caulk along the front and back edges of the threshold to help seal it into place.