Flatten the bag and then rotate it on its side so you’ve got one layer of square knots lined up properly. This will allow you to find the filler cords at the bottom of the bag. Keep those two cords separate.
Make a square knot using the outer cords. Make sure these square knots are tightly knotted together.
After each knot, miss out one outer cord on each side. Move them out of the way and then continue to make square knots. Stop until you reach the middle of the bag.
Gather the filler cords and tie a knot so you don’t lose them.