It depends on whether it's a soft tissue or woody tissue cutting.
Soft tissue.. i.e. this years fresh shoots you select the cutting you want, about 2"-3" and trim it with a sharp knife just below a leaf joint. Pull off the leaves up the stem for about half the length then put into your potting compost and treat as newly germinated plant. Some people recommend a rooting hormone but I had as much success with as without using it.
Some woody tissue cutting can be done the same way or you can layer the plant by pegging down a suitable branch to the ground and covering it with some soil. Leave for a good period of time and with luck roots will have formed and you can cut it away from the mother plant.
