isthis I am no expert but I usually succeed by taking a healthy, young looking stem about 4 inches long, cut off any flower buds, and stick it in a glass of water, making sure that any leaves below the water line are cut off too.
After 3 or 4 weeks, little roots will have started to grow.
Transfer to a small pot - 3-4 inches diameter will do - with ordinary potting compost. Keep on a windowsill over the winter and water very occasionally -if you overwater they will rot.
Put in your border next year after the danger if frost is past. If you are really keen you can transfer them to bigger pots after a few months but it's not really necessary.