The zipper will determine how large the neckhole is. If you want your sweater to be able to go more off the shoulder, you will want your zipper to be longer. Hold the zipper against yourself to decide how long you want it.
Tack the end of the zipper just past your desired length.
Pin the edge of the neckhole onto the zipper. You want to be pinning the outside side of the sweater to the outside side of the zipper, so that when the sweater is right-side-out the seam is inside. Use a straight stitch and a zipper foot to sew the fabric onto each side of the zipper. I put my zipper pull half way down the zipper, and then when I sew up to the zipper pull, I pause sewing to pull it up past the sewing machine foot.
To sew the ends of the zipper to the fabric, I put the foot of my sewing machine up, and used the hand wheel to slowly make stitches. I sewed around the top and bottom edges, tacking over the actual zipper teeth themselves several times when I got to them.
Trim the zipper to length. I used the instructions here: https://indiesew.com/blog/how-to-shorten-a-zipper