The most usual time is August but nothing venture nothing win. Just cut off a piece of green stem about four inches long, remove most of the lower leaves (to avoid too much moisture loss) and any flower buds (flowering inhibits root growth so remove any that appear while its rooting as well). Then put round the sides of a pot of fairly gritty compost. Some people allow the cut ends to dry first, others root them in water altogether so you could try both ways. If you put in compost don't have it any more than just moist as they can rot easily. (This is assuming you're talking about bedding plants not the perennial garden varieties) I've had good results by using barely moist compost but covering loosely with a plastic bag, making sure it doesn't touch any leaves.