You take your old pair of jeans that no longer fit your old pair of butt cheeks.
Cut off the legs and one of the back pockets.
Cut off the ankle cuffs and discard. Cut off the inseams and keep.
Take an inseam, fold it lengthwise and sew it shut (don’t worry about sewing it right sides together, that’d be way too hard to turn right side out again – let your fringe flag fly, sister/brother!). Repeat with the other inseam. And now you should have two strappy looking things.
You take the two pant legs, and right sides facing, sew them together along the ankle lines.
Flip that bad boy over.
Pin and sew the bottoms of the straps just above the seamline.
Right sides facing, fold the unstrapped side (right side in pic) over the strapped side, leaving about 8-10″ of the strapped side free (that will be the lid of the backpack).
Sew that bag shut!
Turn bag right side out.
Pin and sew the tops of the straps to the top of the bag (not the top of the lid).
Sew the pocket onto the lid, and admire your handiwork!