Measure the width of the door. Add a few inches so it will extend beyond the door on both ends. Also, add 1 1/2 inches to allow for fabric seams.
Lay the fabric out flat on a hard work surface. Measure and mark the length you need. Measure and mark a width of 8 inches. Cut the piece out with sharp scissors.
Fold the fabric in half so the right side is on the inside. The width should now be 4 inches. Pin the long edge together with straight pins. Pin one end shut too.
Sew the seams of the long edge together with needle and thread. Sew the end shut too. You can do this manually or with a sewing machine.
Turn the draft guard inside out. Start at the open end and work the fabric up and through with your fingers. You can also use a wooden dowel to push the fabric through the opening.
Place a funnel in the open end of the draft guard. Hold it up and use a scooper to fill it with sand or kitty litter. Leave a 1-inch gap at the top.
Fold the fabric in the open end of the draft guard inward. Pin it shut; sew it shut.