Trim the zipper tape to a length slightly less than the neck edge then sew the zipper to the front edges. Remove the basting thread. Tack the top tape securely to form a zipper stop just below the neck edge.

Take the zipper for a test drive and enjoy your efforts...

You can see I used a 2-way zipper, unzipped from the bottom-up (left arrow) and nicely hidden at the neck edge (right arrow).

Zip ON!!!