This is the fabric I am making roman shades from now. Its linen, it stretches and its got a million points to be pattern matched. One of the roman shades made from this will be a bay window roman in three sections. So one wide section that has two seams / patterned match to make it the width it needs to be and then two small sections that will be the angle side pieces.
Its pinned it in this first photo to get the pattern match, then iron a crease in it and then you hand baste in the ditch on that crease with 1/2 " long stitches to hold it perfect while sewing it. After sewing you take out the long basted stitches.