Prepare your rope
Lay the rope on top of each "seam" or place where the bags meet. Cut the rope into lengths that are about a foot longer on each side than the full width of your sit-upon.
Nylon-type rope can work well because it is waterproof. However, when you start sewing with that rope, the strands that make up the rope can start to unravel, making it difficult to poke through the holes.
If you melt the end of the rope with a lit match, it will make the sewing go easier.