after sewing in the left side of the zip, zip it up, fold the seam allowance in for the right side and line the seam up exactly with that on the left side. Pin in place on the tape.
Zip the zip up again and do a quick check from the right side to see that the seam still matches up with the other side... if the seam has shifted just a bit out of whack then you only have those few stitches to unpick and re-stitch. Better than having to un-pick the whole length, huh?
