I stared at a paper lunch bag and mentally used it for a template. This will yield a slightly larger bag - gotta have room for dessert! I used Ikea upholstery fabric for the two bags on the right. They stand up on their own as opposed to the cotton bag on the left whom I've stuffed with apples for this photo.
Cut the following twice (once of inner lining fabric and once of outer fabric):
Back (including the fold-over flap) 7 x 12 inches
Front 7 x 8 inches
2 Sides 5 x 8 inches
Bottom 7 x 5 inches
Here are your cut pieces. Lay the outer fabric facing down and the inner fabric facing up (both wrong sides are together). I used the same fabric for both the lining and the outside.
Take the back piece and put bias tape around the top three edges (this will be the fold down flap). Stitch into place.
Sew each piece to the bottom. Your lunch bag will look like a cross laying flat.
Good sides together, sew the sides to the front at each edge. It will look like this from the inside:
Take the top edge and sew bias tape over the raw edge. You will be sewing across side one, the front, and side two.
Now attach the back by sewing the back to the sides.
(Wrong side view)
Lookin' like a lunch bag, huh?
Add some velcro to close.
I found the sides gaped a bit, so I added a pleat in each one. Here's a good photo from another lunch bag:
You can do this before or after the bias tape step, or not at all.
(I forgot to say to serge or zigzag all the inner raw edges as you go so that you don't have threads in your lunch.)