I read in an old book somewhere that the hemp plants were "retted", a controlled rotting process in which the fibers are released from the rest of the plant tissue. Nowadays it's done chemically. Hemp fibers are continuous from the growing tip (bud?) all the way into the roots. The fibers are full length, and only had to be twisted together to make a string.
I presume the fibers were waxed or treated in some fashion to repel water, but I don't remember reading anything about that.
Good luck convincing the authorities that you were only trying to grow some bow strings...