Now, the rest of the story...........
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Stitch 1/4 inch you could do a scant 1/4 inch here to be sure to hide the stitching in an upcoming step, but the 1/4 works for me.
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Now you have both long edges of the back sashing sewn to the blocks and 1 long edge of the front sashing. I press the back sashing seams just to set them and w/ some spray sizing. I don't want any shifting and puckering or tucks. Measure the area of the sashing between the seams, this is the width of your batting sash. For me, for this quilt it was 2.25 inches.
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Place the batting so it is nested into the sash and not sitting on any of the side seams adding any bulk to them.
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Fold over your pieced top sash, press in place small spot gluing is helpful, pin matching points and seams, covering your joining stitch.
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Top stitch it down. You don't have to stitch both sides but I think it looks on purpose if you do. This is why I stitch the back sash by machine so I have complete control from the front w/ the top stitching. You will need to do some sort of quilting stitch in the sash b/c it hasn't been sewn to anything yet.
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Those same stitch lines as seen on the back
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I am doing a 12 block layout, so I will make 3 columns of 4 blocks then join the same way w/ a long sashing strip between them.