You match the stripes on the back side of the fold to the front. This requires you to slide or skew the fabric layers to keep the elements of the tartan on top of each other. You also need to baste as you go down the edges of the aprons to hold the skewing. I poke the needle thru the layers to see how much I have to skew. The amount of skewing will vary from top to bottom.
Cheers,
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