I have a bunch of these in my yard. Hoovb is correct, just start topping and shaping. Fertilize to encourage growth. I have a very, VERY large screen that we have to used scaffolds to top, now, along with about 6 tall pillars in the back we need a very tall ladder to get to, to top. I some affectation from the psyllid, but not too bad. The birds like the fruit. I also have several different Pittosporum tenuifolium cultivars, but they are lacier and less dense. If you're looking for a real screen, the Brush Cherry is a better choice. Oleanders aren't really a great choice for a lot of reasons - extremely poisonous (although I have plenty of them as well on my property), but more importantly, Oleander Leaf Scorch, which is untreatable and eventually fatal.
In need of trimming up
Two of the columnar Brush Cherries up against the house (you can see the bark scattered on our paver walkway from the Towhees scratching for bugs).
And, the third columnar Brush Cherry against the back of the house. You can see how tall they get. Our second story windows are quite a ways up, as we have 10' ceilings downstairs.
These are just a few of the Brush Cherries in our yard. Have several more out front that are topiaries as well.
Patty S.
Q