Take the door off the hinges by tapping out the hinge pins. With a partner, lift the door onto two sawhorses so you can access the door bottom completely.
Measure the bottom of the door. Cut the door sweep to the exact measurement, using a hacksaw to cut through the metal.
Hold the sweep to the bottom of the door on one end. Some sweeps have longer ridges on one side; place this to the exterior side of the door.
Press the door sweep onto the bottom of the door. Press all along the sweep until the metal bracketing slips around the bottom of the door.
Hold a small block of wood up to the sweep. Tap it with a hammer to ensure that the metal bracketing is tight against the bottom of the door. Tap up and down the sweep to ensure a tight fit.
Drill pilot holes in the designated screw spaces in the sweep. Screw in the wood screws so that the heads of the screws lay flat and will not interfere with the door opening and closing.
Rehang the door. Open it a few times to ensure the new door sweep is properly sealing the gap under the door. The bottom of the sweep should brush against the threshold but not rub too hard against it.