If you don't have a line of sight direction, then the best way is with a signal strength meter. Contact a local dealer, they should have one.
They may even be able to provide you with a tower location from their network guys (I have an internal online map for our network in Canada) and give you a pinpoint to aim at, which with a few degrees of variability will be fine.
Without these tools, you'll need a bit of tinkering.
I am hooking up a cellular booster. The omni directional antenna and booster helped, but not as much as I had hoped. I just ordered a yagi, and have been trying to locate the closest tower. Wondering how to aim the thing exactly once I narrow down where the closest tower is.
Best way is a signal strength meter it will peak when you are locked on, otherwise you'll have to guesstimate and adjust. It will still work better than omni-directional if a few degrees off (probably good to about +/- 5 deg depending on range), just not as well as dead on of course. If you can get the co-ordinated and then calculate compass direction, then you can essentially 'aim it' with a compass. But it's still best to have a signal meter.
There's really no other way if you don't have it in sight.