The Ink Brush is used differently in the different game modes.
In Turf War, it's highly aggressive. You dash in early, and you spread ink like mad. Once the first 15 seconds or so have gone by, your job is to swim around, and when you see enemy ink, jump out and spam ZR to coat the whole area. It's also easy to zip around people (while under the ink) and pop them from the side or back. It's hard to not be ranked number one with the Ink Brush.
For Splat Zone, if the zones are split, it's easy to hold down one zone by yourself while your teammates attack the second. Place the Sprinkler where it will best keep the zone covered or harass invaders. Then, you can either take the high ground and coat the area (as needed), or you can lie in wait off to the side. It's easy to swim up behind other people that would encroach on your zone and splat them.
The thing is, the weapon takes some practice. It takes around five hits to splay an enemy, too, so be prepared for that. I know it doesn't sound like an elegant approach, but spamming ZR is your friend. No other weapon coats an immediate area like the Ink Brush; it has quickly become my favorite weapon. When used in Turf Wars, I'm surprised if I don't break 1200.
I also HIGHLY recommend Ninja Squid.
