I measured other bags I've hiked with and liked in terms of comfort and enough room to fit my gear. I sketched a general design that had minimal sewing as seams add weight. The pattern is basically three pieces to make the bag. There's two side pieces and one long continuous piece that starts in the front, rounds underneath as the bottom and comes up to the top as the back. I cut and fit a practice bag out of tyvek (although tyvek is a little stretchy under tension so I wouldn't recommend this). I pinned what I could a stitched everything else to really test how the tension would work. I took this apart to use as pattern pieces and cut my final bag material, stitched all the loops and flaps on the back, sealed this from the inside with seam grip +wp. Then I laid out the loops for the shock cord on the sides and front, sewed all side seams (catching the loops in the seams). I seam taped all seams, flipped it right side out and finished the top. Some things I learned: artists tape or painters tape works much better than pins for holding things in place (also less holes). I might consider using dyneema for more of the 'hardware' like the loops for the shock cord - it seems more durable and lighter than the grosgrain ribbon I used.
