Attach spaghetti straps - Sandwiched Method
Front
Pin the straps to the front facing piece or front lining piece.
Use a basting stitch to secure the straps in place.
Once you sew the front neckline, the straps will be sandwiched between the facing/lining and the bodice.
If visible, remove the basting stitch.
Back
Insert straps through the openings. The straps will be sandwiched between the facing/lining and the bodice.
Pin the strap in position and try it on. Adjust the strap length as needed before sewing.
Sew the openings closed on the inside.