Um... anywhere between your pelvis and rib cage. It's wide. I'm 6'2" and can pretty much only put it in one place.
Where it feels right and you get support when you press your belly against it. Where you can squat and whatnot without pinching yourself too bad. Where it doesn't restrict breathing.
Try some different placements if you can put it in different places. I can't. If you're unsure, a pic says a thousand words.