At this point, the bag should still be inside out with two long straps hanging out of each side of the top. Take the two straps from one side and put them through the inside out backpack lined up the the bottom up the bag about an inch away from the corner that they are closest to. Pin in place and repeat with the other set of straps. Pin the rest of the bottom of the bag together and sew across, making sure to sew over the straps to hold them in place.
Take the pins out, check for extra thread that need trimming, turn the bag right side out, and there you go! A drawstring backpack!