First, measure out 2 rectangles, one for the front, and one for the lining. You could also use wadding if you wanted it for a tablet case.
I used a Kipling purse to get an idea of size.
Cut material to size - now the gingham I've used isn't ideal as actually, it was a bin end, and no matter what I did I couldn't get it to look straight. I've used my rotary cutter and cutting mat, as it helps me to line it up, but you could just draw the lines and cut with scissors.
You need to shape the top slightly, I used a wine glass as a template.
Work out where you want the fold to be, and make a very small cut at each edge. With right sides together, sew round the top, up to the cut.
Sew along the bottom edge. DON'T sew up the sides. This will seem very odd and it will look something like this....
Now, this is the slightly fiddly bit.. pick it up and put the right sides of the front together, and sew down the side seams. Do the same with the lining, sew all the way down one side, and on the other side leave a small gap.
It will then look something like this one.....
Turn it right sides out, through the little gap you've left.
Push the lining inside the bag, and this is the final result
Needs a bit of a press, but the general idea is fine.