Sew the lining fabric rectangle to the main fabric rectangle (which is now one piece again, but with darts added to the front), right sides facing each other. Finish off the edge either with a serger, an overcasting stitch, or with pinking shears.
Make a looooong tube (you aren’t sewing the lining and the main piece at the same time, you are making two separate tubes that are sewn together across the top edge.
Iron and hem the outside tube of fabric.
Repeat with the lining tube of fabric.
Here are all of the Blue Dress Sew Along posts-