Moving on to side seams. This is easy. Pin front to back.
Then stitch using elastic or zig-zag stitch. Serge the edges and press your seams to the back.
Prepare sleeves. Make sure to have top notches marked on both sleeves. That notch must match the shoulder seam on the dress.
Sew the sides of the sleeves. Serge and press open. Serge the bottom of the sleeves.
After that hem the sleeves the way you consider feat. You can use a coverlocker or zig-zag. I hand stitched an invisible seam.
Attaching sleeves to the dress. Turn the sleeves so that the wrong side of the fabric is inside the sleeve. Carefully check what sleeve to what armhole goes. Pin the sleeve to the dress. Make sure that the seam on the sleeve matches the side seam on the dress; and the notch on the sleeve matches the shoulder seam on the dress:
Machine stitch using elastic or zig-zag stitch. Overlock the edges. Press the seam to the sleeve.
