I cut down the fabric to 9.5x13.5 to allow for the seam (or any creative changes) and then used that block to measure out the twill lining that would go on the back. I serged the tops of the curtain and liner together. Then I pinned and hemmed the bottoms separately to the right length (13 inches) and sewed them separately. Finally I serged the sides together.
And no itty-bitty curtain is complete without a ruffle at the top. I used my new ruffler and set it to 12 (pleat every 12 stitches). It wasn't "ruffly" enough for me so I ran it again at 6. Still not what I wanted, but it was okay and the dark fabric hid a multitude of sins.
I attached it all with velcro to the curtain and then to the door over the metal casing. Now that the cat door is slammin', the paint on the door really looks like it could use some help. It's always something, no?
Anyway, go dress up your furry friend's digs. He'll love you for it.