Open the door, grab the door knob on either side of the door and lift up; this helps align the door with the hinges on the frame and helps the door better fit with the weatherstripping you'll install later. If necessary, screw down the hinges with the screwdriver to secure the door. Close the door, and measure across the top of the door and down the side of the jamb. Jot down your result.
Mark and cut the measurement on the weatherstripping. You'll need two pieces that reflect the side measurements and only one for the top measurement, leaving you with three cut pieces of stripping. Press each piece into place on the top and sides of the jamb, and nail them into place, spacing the nails about 2 inches from each end of the jamb and 12 inches thereafter.
Pull on the door after you've nailed down the stripping into the jambs to ensure a maximum amount of contact between the door and the stripping. Look at the bottom of the closed door; if you see small amount of light or feel a draft, install a new door sweep to properly seal the door. Measure the bottom side of the open door, and jot this number down.
Use the hacksaw to cut a sweep that conforms to the door's measurement, and screw it to the bottom of the door facing inside the house. The small gap you see is natural; however, nail a small 1- by 2-inch piece of furniture padding to the door frame to fully seal the house and door.