Removing old threshold:
Push back your door to its limit, as far as it goes. Use a circular saw to cut the existing threshold in half. Be very careful about the floor beneath. Use a crow bar to pull up the strip. If the old threshold was a metal one, all you have to do is to unscrew it. Clean up the area for dirt that has accumulated over the years.
Measurement and preparation:
You need to measure the length and width of your doorway between getting the new threshold strip. Another measurement that you need is the distance between the bottom of the door and the floor. Use a measuring tape for this and note them on a paper with a pencil to take to the hardware or home improvement store for the new threshold.
If you do not get an exact width for the strip, mark your door width on it and cut it using a saw.
Apply a good layer of caulk on the floor that the threshold is going to cover. The caulk will act as air-tight seal between the threshold and the floor, thus preventing heat losses.
Installing a metal threshold is relatively easier than installing a wooden one. Metal threshold kits comes with screws. So all you have to do is to fasten the screws in the right holes and your job is done.