to make a pattern for your window seat --get your cushion for the window and measure the top of the cushion, length and width take that measurement and add 1/2 inch seam allowance all around.once you get this measure cut out two times for the front and back of the window cushion. Now you need to get the measurements for the band, length and width (which has four sides to it) add 1/2 inch for all four seam allowance. Make sure you leave an opening for inserting the cushion into the finished product.
Let me know if this helps.