As far as I recall the rule is to try to trim (at a slant so rain falls off) above a bud, so the rose can grow from the bud instead. But in my experience a) there's never a bud low enough, b) no matter what you do the rose will give up on the branch you wanted to keep and yet still grow off in a direction you don't want, and c) sometimes it's better to just shape it and let it sort itself out.
The experts at Wisley tried 2 different methods one year. One lot of gardeners did some beds in the old-fashioned way- removing excess growth from the centre of each plant, trimming to outside buds, creating a goblet-shape
the other lot just got some hedge trimmers and treated the bushes like hedges. The ones in the latter group bore more leaves and far more flowers the next year.
The general recommendation around here in Derbyshire is to get your worst enemy to prune your roses !!! I suppose the idea is that whatever you do you cannot go wrong.