Edited to add my lining method, see below...
Today was Sewing Bee part two, hosted by the lovely and talented Jessica. (Thank you so much for having us over!) I made a backpack start to finish and I only ripped and re-sewed about 1/3 of the seams. Part of my difficulty was due to adding a lining not specified in the pattern and figuring out how to make that all come together nicely.
In the end I am pretty happy with it, but the brown thread, although it matches the brown fabric, looks an awful lot like black against the pink.
Pattern from Lotta Jansdotter Simple Sewing, brown Kona cotton, Amy Butler Lotus Collection camel/full moon polka dot fabric
I figured that I could add a lining if I could make it just big enough to catch in the seam of the strap casing. So I cut the lining the same size as the bag piece and then fiddled with it until I found the right height and cut the lining down. I think it's about an inch shorter than the bag. Any longer and the lining would have intruded too much into the strap casing.
(I thought about making the lining the same height as the bag and folding it down along with the outside of the bag making a double-layered casing, but I figured that would be too thick.)
After I sewed up the sides of the bag I turned it right-side out. Then I sewed up one side of the lining and sewed from the fold up a couple of inches on the other side of the lining. Then I inserted the lining into the bag. I folded down the top of the bag for the strap casing and pinned it in place catching less than 1/2 inch of the lining under the strap casing and sewed it together.
I threaded the straps through the casing and pinned them to the bag making sure they weren't twisted. Then I turned the bag inside out through the open part of the lining and sewed up the side seams to attach the straps. Turn it back right-side out and hand stitch the lining closed.
Viola, almost Simple Sewing!