This is a quick, cheap project that can save you money in the winter months.
Step 1: Measure and Cut
Next, pick your fabric. You'll need something fairly thick. We used utility grade linen.
Now, cut 2 pieces of fabric 8" wide and the length of your door... 36" in our case. After you cut, you'll need to lay them on top of each other and pin together. If your fabric is one sided, you'll want the pretty sides on the inside facing each other, so the ugly sides are facing out.
Step 2: Sew
After you sew, trim the sewed edges.
After trimming, turn inside out. Now the pretty side of the fabric will be facing out, and your ugly side will be on the inside.
Step 3: Divide
Sew the length so you have 2 tubes with an open end. One of these will be filled, the other will be the flap holding it to the door.