Quioting the RHS Encyclopedia of Gardening:
In mid to late summer take healthy shoots of the current season 's growth, cutting just above a node. The shoots should be still soft at the top and firm at the base.
Remove any side shoots. Trim off the soft top of the cutting and remove the loser leaves. Carefully cut away a strip about 1 - 1 1/2 inches long from one side of the base. (This stimulates rooting). Dip in hormone rooting powder if you use it and insert the cutting into a pot. Put in a cold frame if you have one. (Not sure if these root better in a propagator) Do not allow to dry out but don't overwater either.

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