BBC gardening website says this:
Select a healthy shoot from the current season's growth. The number of leaf pairs on the shoot will directly correspond with the number of cuttings you can take.
Use a sharp garden knife to cut the stem just above every pair of leaves - take care not to damage any buds.
Remove one of the leaves from each cutting. This will help to reduce moisture loss from the cutting and avoid the cuttings being overcrowded when they are potted up.
Dip the base of the cuttings in hormone rooting powder. Fill a pot with free-draining compost and insert the cuttings around the edge, so they do not touch.
Give the pot a thorough watering using a watering can with a fine rose. Then place the cuttings in a plastic bag or propagator, which will speed up rooting.
Rooting will take several weeks, a sign is when the leaves begin to perk up. To check on progress gently remove the cuttings from their pot.
Once the cuttings have rooted, fill 7.5cm (3in) pots with multi-purpose compost. Pot up the cuttings individually, making a good-sized hole for the roots before firming in.
Water the cuttings well and grow them on in a cool place, such as a cold frame. By the following spring they will have grown large enough to be planted outside.
Once planted, your cuttings will quickly establish, covering their support and producing masses of beautiful flowers