I've added them after the fact, simply sewing up a liner, then rolling and sewing the lip of the bag to catch the top of the liner. Nothing fancy, but I liked the result of stiffening the mouth of bags made of softer or thinner leather so much that I know roll the tops of most of them.
The back of the liner I've "captured" with everything from an added ply to the bag flap, to sewing a single strip of leather across the back, to sewing in a free hanging pocket across the back of the bag, which also secures the lining.
All work, so it kinda depends on what you want.
Sorry, but I'm on the road with no access to my bags so I can provide photos.