Not easy - leaf or bud cuttings in summer, but the easiest is probably hardwood cuttings in autumn. Unless it's a variety of C. reticulata, when it can be layered, but most these days are varieties of C. williamsii because they're hardier.

You need to take cuttings from vigorous growth in summer. Take 4 or 5 inch cuttings and use hormone rooting powder. Put them in a pot of leafmould and compost mix and then in a propagator with botttom heat. This heat is the secret! They should root in a few weeks, but wait until they have good roots before potting on.