Ok - I have to start by saying that lining this bag was challenging, but I made it more difficult because I tried to do guesswork with the measurements, so there was a lot of re-cutting and re-stitching.
To line this type of bag, you’re basically creating a “box” out of the fabric, then you stitch the top edges of the lining to the inside of the bag.
The key measurements are the width of the entire bag (measured across one long side); the height of the the bag, and the width of the opening from front to back.
I tried to make some pictures of how I sewed this…but they are not too clear… Next time I do this I will take pictures at each step.
The main principle is that you sew a rectangle right sides together, leaving an opening (top of the bag); then you stich across the bottom corners of rectangle to create the box shaping at the bottom of the bag. If you want to include pockets, position them and sew them on to the right side of the fabric before you do the side seams of the rectangle.