This tutorial explains how to sew the armhole facings of the Ethel Designer Top
Neaten the outer curved edge and short edges of the armhole facings with an overlocker/serger. With right sides together pin the armhole facings to the armholes following the notches. Once in place stitch from the step notch at one end to the step notch at the opposite end, be sure to backstitch at each end. Clip into the seam at each step notch and turn the facings out to the right side. Understitch close to the seam on the side of the facings and press the facings to the inside of the body.
Pin the armhole facing to the inside of the body and topstitch around the overlocked/serged outer edges of the facings.
With right sides facing sew the front and back side seams together, clip the corner of the top edge of the side seam so it isn’t showing. Neaten with an overlocker/serger, press towards the back, then secure the top of the side seam to the body with a bar tack.