: If for example you were making a rectangular 10in x 20in bag
: out of one pc of 20in by 20in material(just for explaining), fold the
: fabric in half to get a double layer 10x20

so far the ideas mesh, but at this point you could diverge. Get another piece of fabric 3 to 4 inches wide and 12 to 18 inches long. Fold this to make a piece 2 by 18 ( assuming you use the larger measurements). Line up the fold on this piece with the fold on your bigger part, and glue an inch of the length of this to the bigger piece. when the glue has set, fold the big piece and the long skinny one will fold too, giving you a rectangular bag with a tail. Heat seal or glue the perimeter of this and you will construct the bag and fill tube at the same time. After inflating the bag by mouth, fold the flat fill tube 3 times and hold it folded with a couple of wood clothes pins, or a piece of duct tape.

PGJ