The easiest way to do it is by layering the plant. This involved taking a stem that is still flexible and bending it down to the ground. You then slightly nick the side of the stem closest to the ground and bury the nicked stem in the soil. Keep the area moist and within a couple of weeks to a few months, the stem should have rooted and can be dug up. In your zone, if you did it now, I would probably wait until next spring before digging the new plant up and severing its stem to the parent plant.

Another option is to take cuttings and attempt to root them with rooting hormone, a fairly quick draining potting mix containing lots of perlite, and maintaining high humidity levels. This can be tricky depending on the type of clematis you are attempting to root but can work.

I personally would try layering if it were me. Try Googling layering clematis and you will get some hits with pictures to demonstrate the procedure if you are still unclear on it.