The 2 major parts of the bag were ready to be joined together.
First, I stitched the lining base to the exterior base. This is easy.
I turned my exterior bag inside out and placed the lining bag base and exterior base wrong sides (WS) together. I stitched along all 4 sides with a 5 mm seam allowance.
I folded the exterior bag back, outside in, over the lining. The lining is now inside.
I stitched the lining and exterior top edge and a part of the side seams together, along the seam that was created by joining the zip top panel and the main body pieces. It can easily be machine stitched with a 5 mm seam seam allowance. Just keep away from the original stitching line.
Then, I stitched - by hand - the folded edge of the lining to the zip.
I attached the adjustable strap to the bag, following the guidelines on p.106-107.
Some final pictures:
Ready to go!