First I did the back and front. Sew four bags together with a flat overlap seam, zigzag stitch on the machine. Zigzag two bags together for the side, make two. Then join the four pieces together with running stitch seams on the outside, so you have a bag with no bottom. Next join two bags with zigzag seam for the bottom, trim to size and pin into place. Sew together. Fold top edge over one and a bit inches and pin. Make two handles out of two bags, you have to open bags out lengthways and cut two strips, fold lengthways, sides to middle, and stitch. Do two rows of running stitch around the top edge half an inch apart and attach handles while you are doing this.
Overall dimensions are 14.5 inches tall, 15 inches wide, 7 inches deep, it will fold flatish when empty. It seems quite strong because it is double thickness, although I would use it for lighter items such as toilet rolls and cereal boxes, rather than heavy vegetables. That way it would last a lot longer. I will try it out on my next shopping trip.