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You should now have an opening in the side of the fabric the length of the zipper. Sew the opening closed.  Hold the zipper in place with hand basting or transparent tape. Attach the zipper foot. When you get to the end, square off and stitch across to the other side of the seam. Square off again, cross the seam, and complete the row of stitching where you started.
Sew the three sides without a zipper using a ½-inch (1.27-centimeter) seam. Press the seams open. Turn the duvet cover right side out at the zipper opening. Stitch from the corner where you put the first zipper markers. Backstitch to secure the thread. Skip the zipper hole and sew the other side in the same manner. Set your sewing machine to machine baste. Press the seam open. Center the zipper teeth over the machine-basted portion of the seam. Put the sewing machine back on its regular stitch setting. Sew on the right side of the fabric close to the zipper. Repeat the process on the other side of the zipper. Remove the machine-basting thread and the hand basting and/or transparent tape. Open the zipper to test it.