Hi, I would follow PittsburgPams directions and measure ends and middles, and also put the stretchy fabric on the bottom as you sew. Pin each piece too, pin each end and at least once in the middle, don't take out the pins till you get right up to them. This will stop you from stretching them more. Always have your sashing and borders on the bottom as it creates less waving. Always measure and pin, don't just sew a sashing piece on the block and then cut, you will get stretching that way.