To form the bag, especially in smaller scales, it is easiest to work against a solid surface. Trim pieces of scrap craft wood, stir sticks or similar wood to make a piece which is as thick as the side edges of the bag, if you use more than one strip, make sure at least one end of the strip is cut straight and square for when you fold the base of the bag.
To fold the center seam line, line up the folded edges of the bag so the creases in the side fold into the center of the bag. Lay your craft wood under the center back seam of the bag, apply glue to the larger side of the back, and glue it to the other side. Allow the glue to dry.