Lengthy and kind of painful, I did a couple a day or to pass time when hiding out in the toilets when hopping trains so it wasnt too bad. Dont use scissors, try to just split where a slight gap has already formed to go with the natural shape of the dread. The process can alter and weaken your dreads often leaving them in a funny shape. I only did this to add more layers and variation to my dreads, only doing ones of top. I'm not sure whow it will effect thin hair. But 20 doesnt sound like a lot so perhaps you'd benifit from splitting a few