I have prepared a bowl of tow to mix into gesso grosso. The rope I used was jute, with a synthetic strand bundle (the dark green bundle of fibres). First, I cut the rope into lengths of around 10cm and then unwound it to remove the synthetic bundle.
I tried cutting the fibres with a knife but found snipping it with shears much easier. I cut the fibres into pieces between 2mm and 10mm long. It took a couple of hours to get this half bowlful.