Sew all around the body of the bag, an inch or so below where it would scoop if you drew a line from there to the edges. Do not sew the arms/straps at all. Then sew the "neck"/the inside of the scoop. Then you turn it out and stuff the one bag into the other so that right sides are facing out.
On both sides fold the edges in a quarter of an inch and press and pin. Stitch a quarter inch down both sides.