i divide my comfrey all through the growing season, i have less susccesful results in the fall.
dig up as much of the thick root as you can, it should be about as thick as your finger. dig it up and cut them into 2 inch sections. lay flat in a tray full of sand and
compost. cover lightly and dont let it dry out. once rooted either place where you want them. or in a pot for a week or so until more roots establish. do not leave in a pot long term, comfrey HATES pots.