You will want to cut a new pattern piece out of lining with both outside edges that are the same instead of one side (the side that connects to the horn) that slants up. You can do this by splitting the pattern piece up the middle or by folding the original pattern piece up the middle, then place the fold or the cut that you just made on the fold of the fabric so that you are cutting a piece with a mirror image on both sides. After doing this, you will want to alter the pattern piece to fit the size window or the size swag that you want to make. This is called a mock and to alter a pattern it may take a few mocks before you get the size just right. Good luck !!