I think you need to overlap the pattern by the amount of the seam allowance (doubled). For example, if you are sewing with 1/2" seam allowances, each side of the zipper will require 1/2" of the pattern that will not show when you are finished. You would cut one 2.5 by 14" piece for one side of the zipper, then overlap the pattern by 1" in order to cut out the second piece. You would need to mark these pieces so that you actually sew the correct piece to each side of the zipper.