Sewing on the sides of the backpack is probably the most difficult part of the backpack. You are sewing a straight edge loop to a curved flat piece. I start by using clips to hold together layers at notches and marking. I don’t clip the layers together anywhere else.
Starting at the center bottom I sew 1/2″ from the edge all around the front. When I get to the curves of the bottom corners I clip in 3/8″ along the side piece so that I can curve it more easily before stitching. When you get to the end of the stitching you may find you have a slight pucker. If you are finicky then you will need to rip out the stitches and ease in the fabric. If you are not finicky then leave any small puckers, when you turn the backpack you probably won’t even notice it and it’s on the bottom. You could stay-stitch along all seam lines if you wish to prevent it all together, but I have only even seen a 1/16″ of extra fabric.
The seam is 1/2″ and the bias binding is also 1/2″, if you want your binding to cover your seam stitching then I suggest you trim your seams down to 3/8″. Contrasting thread will be visible like in all of my backpacks.