Step 1: Clean out a bag.
Stitch along the bottom of the bag 1/4" from the bottom.
Turn bag inside-out and sew along the bottom 1/2" from the bottom. This gives you a French seam which encloses the cut edge and helps make it more durable.
Stand the bag open end down on the table and take the two corners and make "dog ears". I measured 2 1/2" from the corner and sewed across the corner.