Place a bag base right side down on top of the right side of one of the outer bags. Make sure the cut out corners are at the top. Sew across the seam.
Press the seam down towards the base. Then top stitch along the seam line on the base side of the seam. Repeat for the other bag outer.
Place both outer bags with the right sides together. Sew down the sides and across the base but DON’T SEW AROUND THE CUT OUT CORNERS. (I can’t tell you how many times I have done this while listening to something REALLY interesting on the radio!)
Press the seams open. You can also top stitch either side of the seams. It can be a bit tricky dealing with the bulk of the fabric but do persevere, as it will hold the seams flat when you come to sew the lining in. Sewing down one side then back up the other is an easier way of doing it.
Now to flatten and sew across the corners, make sure to try and get the side and the base seams right on top of each other.
Make sure to reverse at the start and end of the seams to lock in the sewing. This is what creates a nice strong corner to your bag.
That’s the bag done, now for the lining.