I cobbled the heavy weight fabric together from a cheap cushion cover and my old coat that I loved but was a bit worn around the edges.
The lining fabric was also from a cheap cushion cover.
I hope the steps are clear - feel free to ask if not!
Pinch the bottom corners diagonally and sew flat to give the bag some depth. Do this to both the heavy fabric corners, but only one of the lining corners (ie leave the hole).
Make straps by hemming strips of fabric.
Turn the bag the right way out, and pin the straps to the outside as shown. Leave the lining the same way out as you've sewn it.