Measure the width of the door's bottom edge using a tape measure. Purchase a door sweep that is at least as wide as the door. If you have to purchase one that is wider, use a hacksaw to cut the metal part of the sweep and scissors to cut the vinyl part.
Close the screen door and position yourself on the same side to which the door swings open.
Hold the metal part of the sweep with the screw holes against the bottom of the door so it is straight. Ensure that the sweep part hangs below the door enough to cover the gap but not too low that it interferes with the door opening and closing. Mark the screw holes using a pencil once the sweep is in the proper position. Set the sweep aside.
Drill pilot holes on each of the pencil marks using an electric drill with a 1/16-inch steel drill bit installed. Drill the holes to a depth of about 1/8-inch.
Fit the sweep to the bottom of the door and align the screw holes with the pilot holes you just drilled. Insert all the screws by hand and turn them once or twice. Use an electric drill with a driver bit installed to tighten the screws completely.