For the straps, sew the light green fabric pieces to the dark green fabric pieces. Fold over and sew 3/8 inch from the edge. Cut ends at an angle as shown and sew one end closed. Turn the straps inside-out and stuff with polyfil. Pin edges to the backpack front 3.5 inches from the zipper on either side (3 inches for a child's backpack). Carefully try on and adjust as needed. Sew straps 1/8 inch from the edge.
Next pin the back panel on and sew 3/8 inch from the edge. Unzip and turn inside-out. Sew on the button and you're done!