Stitch across the bottom of the outer bag. Stitch across the bottom of the lining but leave a large gap of about 15cm to turn the bag through later. You will only need to stitch about 4cm at either side of the bottom to leave a large enough gap.
Now for the bottom corners of the bag. Open the corners and separate the seam allowances out flat. Squash the seam allowances flat together with your fingers and match up the side seam with the bottom seam.
Stitch straight across the corners.