You will need
Cut the centre back
Place the zipper on the edge of the sweater upside down with the wrong side facing up.
Pin the zipper to the sweater edge. Unzip zipper when it is secured.
Use a zipper foot, to sew the zipper to the sweater. Help the sewing by pulling the sweater gently when it is needed. Repeat the same steps for the other side.
Topstitch the edge for a finish touch.