Last year I tested a number of germination methods using around 120 test cases on various clematis species. The link below is the full report. Clematis seed germination study

I would summarize some findings here.

Because most clematis
seed takes quite a while to germinate, it is probably a good idea to
start the process as soon as you get the seed.

My standard see germination procedure is the baggy method. Take a baggy, add some paper towel, seeds and a bit of water. Over time keep the towel just moist. When seeds germinate - remove and pot up.

I compared this to a method where you put seeds in water, and leave
them there until they germinate. I also tried the water method with
seeds where the outer coating was removed.

Other test checked cold stratification, darkness, leaving on tails, and using GA3 to speed up germination.

120 test cases were examined.

Summary:
All tested seed germinates warm. Most do not need cold stratification or darkness.

The baggy method works better than the water method. Seeds with the seed coat removed, in the water method, mostly rotted.

GA 3 was a requirement for some species. It is possible that if I wait
longer these seeds will germinate without GA3, but all seeds have now
been in the process for 8 months.

The following are some specific recommendations:
C. fusca - easy to germinate warm
C. ispahanica - easy to germinate warm
C. crispa - easy to germinate warm
C. virginiana – requires GA3 and warm
C. pitchri – warm and may benefit from GA3
C. orientalis - easy to germinate warm
C. viticella – warm and dark
C. integrifolia - easy to germinate warm
C. stans – try warm for 2 months and if no germination give a cold treatment
C. heracifolia - easy to germinate warm