This should be the last part
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Press seams however you like, alternating (lefties outies, righties innies or similar will help in joining the zig zags)
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Trim two sides, keeping an eye on the 45 degree line and final size requirements, there should be just a tiny amount of trimming and the dog ears.
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Turn rectangle and trim again to final size.
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Enjoy your beautiful zig zag border (or whatever you want to use the units for)
Don't be intimidated by all the steps it really is easy, I just tried to be as detailed as possible.
Remember you are stitching bias edges so starch (or my favorite sizing) and don't pull them through the sewing machine, or pull when pressing.