I made the one with a sun for my son, and the one with the skull for my daughter for Christmas.

On the skull backpack, I used silver glitter fabric paint to make silver dots at intervals around the pack.

For the straps, because I didn't have anything around the house useful for that purpose, I single stitch crocheted the shoulder straps and sewed them INTO the backpack seams. I also single stitch crocheted the draw string, changing to double stitch where the two strings met when stretched out to ensure the draw string didn't get lost or partially stuck inside the seam.