We grow this shrub in both England and France - and don't really do much pruning of them at all, just a light 'manicure' to keep them tidy - they seem to be very easy going and undemanding. I have heard, like Lin, that you should not cut into old wood or you risk killing the plant.
I did a quick internet search and found this
https://www.gardeningknowhow.c...
which seems to say that it's important to start pruning when the plant is young, and gives advice on how to prune. I hope yours flourishes!
