Even better, I was able to use things I already had on hand! I had some leftover canvas from my homemade shopping bag project (or rejected project - so no worth it when you end up getting them free half the time) that was just perfect.
I ended up with two pieces of fabric that were just the width of the door, and about two feet long. Probably more fabric than I needed, but I just split what I had sitting in my box. I folded each piece in half and stitched up one of the long sides and one of the short sides.
Then I turned it inside out, and sewed two rows of stitching down the middle (this is the part that goes underneath the door). For the draft guard for the front door, I stuffed both sides with some leftover stuffing.
For the back door, I used stuffing for the part on the inside of the house, and crushed packing peanuts for the outside. I used the packing peanuts for two reasons: 1. I didn't have enough stuffing to do both sides, and 2. there's very little overhang over the backdoor, and I figured stuffing would end up a soggy, disgusting mess when it rained.
Super easy project, and will hopefully save us a few bucks on heating this winter!