Hmmm, I always have to scratch my head a bit and think it through, but basically I make the liner like the bag, less the zipper top piece. When I sew the liner I leave an unsewn seam in the bottom of the liner. Then I turn the bag inside out and sew the liner to the top seam ( around where the zipper top piece was inserted) and then I pull the bag right side out through the gap in the liner seam. Finish with a bit of hand sewing where the bottom liner is open and you are done. Pretty slick, but you sort of have to think about it a bit.
Next one I do I will take some pictures... May be awhile since I just finished up with bags for Christmas presents. My family are cyclists and they get bags this year!
Nice work! I really dig that fold out liner, it makes for a nice tidy interior, but I'm having a bit of trouble wrapping my mind around how to assemble something like that. Do you have any progress photos to help the feeble minded?