The backpack construction is of two modified rectangles joined by a zippered gusset. Take your main fabric and cut two rectangles of the approximate size you want your backpack to be.
Fold them in half lengthwise and round the corners to the finished shape.
If you want to applique or embellish the front of your backpack, now is the time to do so.
Cut gusset pieces out of main fabric and lining. They must be as long as the circumference of your bag. Cut the gussets in half, then sew together again. I sewed two zips in, one in each end, and stitched in between.
You will have a long strip with a zip in each end.
Make shoulder straps from lining and/or main fabric. I attached a short piece of elastic at the bottom of each strap, rather than making them adjustable.
Hold bag front and lining, WRONG sides together. Do the same for the bag back. Pin straps to back of bag. Baste. Add a loop for hanging if desired.
Pin gusset to front and back and stitch. Bind seams with contrasting binding.