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!
Cut out two equally sized pieces of lining fabric slightly bigger than the size you want your bag to be, and cut out 1" squares from the bottom corners.
I made the front and back panels from strips of patterned fabric, but you could skip this step if you'd like a plain bag.
Place the 2 panels right sides together, and cut to the same size and shape as the lining panels. Pin and sew the heavy fabric panels together down each edge and along the bottom. Repeat with the lining panels, again with right sides together.
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.
Place the bag inside the lining as shown, making sure the straps are well out the way of the top of the bag. The ends of the straps should overlap the top so that they can be sewn in place.
Sew the lining to the outer bag around the top.
Pull the bag inside out via the hole you've left, and see your new bag emerge! Sew the corner closed by hand and push the lining into the bag.