You can propagate clematis in several ways but May is the time to take semi-ripe cuttings.
The critical test is to bend the shoot and discover where soft growth runs into semi-ripe growth and cut two nodes below this point. I root at nodes like you would with any other cutting but internodal cuttings also work. Cut off and discard the soft top just above a node and bury the whole cutting in deep pot up to this point. If there is more than one buried node so much the better. Use free-draining peat-free compost or something like perlite or vermiculite.
Please don’t use a propagator or a polythene bag but put the moist pot somewhere shaded and sheltered outside or in a cool shaded greenhouse. Plastic and too little air movement can trigger fungal attack. I suppose fungicides could be used but I can’t be asked. Rooting compound does little except calm the inexperienced. You can cut off half the leaves to reduce water loss. If you don’t the cutting will do the same job by shrivelling some foliage.
Not every cutting will root. Leave the cuttings to grow on and pot-on if needed. The rooted cuttings need a long season of growth to make an overwintering bud and without it your triumphal plantette will not reappear the following year. Hence May and not June. Hence the pot which you might eventually move to protect as winter sets in.
