This can be done in two different ways:
1. In the fall, dig up the whole plant and cut the roots in about 2 to 6 inches long. This way is good if you want a large number of cuttings.
2. Dig a spade alongside the clump and pry upwards and outwards if you want to harvest just a few roots.
It gets a little more involved after this step. The thickness matters according to which plant you want to propagate.
This said, you then mix equal parts of peat moss, sand and soil. Press them down and cover them with about one inch of sand and water generously. Put them in a cold frame for the winter.
I have done this with thousands of root cuttings when I worked in a plant nursery.
Since I don't know exactly the plants you want to propagate, so I can't get more specific as to the diameter of roots, etc, but this information is easily found at the library, Or you can post exactly the plants you want to propagate, etc.
Good luck

Can this be done in the early spring, like late March as well? I am trying to do spiderwort by root cuttings. Thanks