When do you "prune" the long bougainvillea branches on the fairly new plant?

i want my 4 month old bougainvillea plants to grow fuller not taller. how and where i trim them to do this?

When do you "prune" the long bougainvillea branches on the fairly new plant?
I am in California and my experience is that they sulk from transplanting, try not to disturb the roots and then i don't prune until I see new growth,,, which can be quite a while..I have seen them for 5 yrs not put much on top but then take off in the 5th yr which tells me they are putting down roots before the add on top.