I used Euroflax Linen for the cords because it is strong and durable.  I made two cords for each bag.  To make one cord, I cut lengths of linen 2 1/2 times as long as the circumference the circle.  I twisted the strands together until they were spiralling around each other, then tied a knot at one end. At the other end I fastened a bodkin, a little tool made especially for threading things like this.

Beginning at one of the holes in the circle, push the bodkin into the tunnel and work all of the way around the circle. Pull the cord through so that the beginning and the end of the cord are even.  Thread the second cord the same way, starting at the hole opposite the first.