Replace Your Weather Strip
Older wood doors usually rely on a non-adjustable threshold to keep the weather out. If your old door doesn't seal tight against the threshold, you're wasting energy. You could screw a surface-applied weather strip to the face of the door, but a door-bottom weather strip is a less obtrusive way to create a good seal.
The door bottom we're using is available at most home centers and hardware stores. If you can't find a door bottom that's smooth on one side, you can slice off the barbed flanges from bottoms designed for steel or fiberglass doors.
Cut the bottom of the door to allow enough (but not too much) clearance to install the new door bottom. The goal is to create an even 3/8-in. space between the top of the existing threshold and the bottom of the door. Close the door and measure the largest gap between the door and the threshold. If the gap is less than 3/8 in., calculate how much you'll have to cut off the bottom to equal 3/8 in. Mark this distance on the door at the point you measured. Then use a scribing tool to extend a mark across the bottom of the door.
Remove the hinge pins and move the door to a set of sawhorses. Mount a sharp blade in your circular saw and cut along the line. Protect the surface of the door with masking tape. If you have a veneered door, score along the line with a sharp utility knife before sawing it to avoid chipping the veneer.
Cut the door-bottom weather strip about 1/8 in. shorter than the width of the door and tack it to the bottom of the door with a staple gun. Rehang the door to test the fit. If it's too snug, remove the weather strip and trim a bit more from the door. When the fit is perfect, remove the staples and mount the weather strip.