Sew the back neck guide to the wrong side of the inner back neck seam and the back armhole guides to the wrong side of the inner back armholes.
With right sides facing sew the back neck seams together, press seam towards the inner back. From the right side, understitch close to the seam on the inside back neck.
With right sides facing sew the back armholes together, press seam towards the inside back armholes. Turn out and from the right side, understitch close to the seam on the inside back armholes.
Sew the front armhole guides to the wrong side of the front armholes.
Trim away any excess from the neck and armhole guides.
With right sides facing, fold the front neck at the shoulder seam, sandwiching the back shoulder seam in between the two front shoulder seams. The armhole edge of the back must meet at the front armhole seam allowance for a neat finish. Stitch from the neckline to the armhole edge, clip the corner at the neckline.
Once the shoulder seam is sewn, pin the two front armholes together and stitch. Press the seam towards the inside front. From the right side understitch close to the seam on the inside front armhole.
Open out the front and back bodies. With right sides facing pin the front and back side seams together, matching underarm seams and notches. Sew from the underarm to the hem for the top layer then repeat from the underarm to the hem of the inside layer. Take care not to stretch the seams when sewing.
Press the side seams open.
Neaten the hemlines of the inner and outer bodies with an overlocker/serger. Turn up the hems at the notch and stitch. If preferred, double turn both hems and stitch a narrow hem.
Lightly press your new top, do not press the cowl fold line.