When I say "I" I mean my dad divided up my plants while mum and I had a cuppa 8)
Rinse the plant gently in cold water to remove excess mud etc. Don't let the plant dry out while you work on it.
You can slice or break off the 'eyes' on the discarded rootstock to propagate more plants.
Cut the rootstock with a sharp knife. This severs the new growth and creates a fresh palnt. Remove the long anchor roots from the new plant, making a clean cut close to the crown. Leave the smaller roots intatc.
Removing the old roots encourages the plant to grow new, stronger, ones and will stop the old roots rotting and infecting the crown.
Trim any leaves from the top of the plant.
Carefully place the newly prepared crown on a perforated container with peat.
Add a layer of small stones or gravel on top to stop the peat from floating free when the container is submerged.