Method of sewing zip to a garment.

Attach facing to the neck line. (I have shown only the back side of the garment here as it is difficult for me to show both the sides that is front and back.) Press under the seam allowances of the back opening
Close the zip. Mark the length of the zip on the garment. Stitch the seams together on the fold line till the mark and press the seams open
Pin the zip to the edges to the folded seam allowance such that the center line of the zipper coincides with the edges of the seam allowances and the zipper tab end should be placed ½” below the neck line seam. Trim off the zipper tape just below the neck line. Baste the zipper to the garment. Do the same thing for the other side also
Using the zipper foot machine stitch the zipper to the garment. . Fold the neck facing and finish the neck line using hem stitch