To make the gusset fold inward, create a stay with a window, a stay means literally “stay here”. Fabric or any material follows the path of least resistance. So you would take you backing (Pellon) and in the middle you would cut a window out. The Pellon should be cut from the lining pattern and glued with rubber glue to the material.
I would also taper or angled the bag this will give the bag more shape. Look at the cut out when Linda makes the window for a zipper pocket in shorts, now apply what you see to your gusset. The cut out is called a window.
To make this more effectively you would need to learn structured techniques which aren’t taught in BH1-2.