As far as my experience goes, just like any other cutting.. Chop a bit of fairly sturdy growth off, around 15cm. Chop top off. Stick in soil/water, wait. And wait. And wait.
When I was in the French Alps last Sept, I was delighted to find that a restaurant had stuck some Passiflora (violacea?) foliage in a vase on the table. I swiped it, wrapped it in a food bag/cottonwool. And then we drove 15 hours back. And it's been sat in a pot since then, and only just burst into growth.
I'd guess the main thing is to try and get older growth rather than new floppy shoots. If it's caerulea you have, Americans reckon it's hardy to at least -20 C. According to them, us Brits mollycoddle our plants..
Melanie