This one is pretty much a two-step process. There's definitely some argument as to precisely where you need to place your garage heater, but almost everyone agrees that it is going to work best if you put it in one of the corners of your garage. Try to make sure that the heater is aimed at the garage door at a 45-degree angle so that you get the most possible coverage with the least possible waste.
The only other issue here is going to be the location of your heater in relation to the location of your thermostat. If you are looking for efficient functionality, your thermostat is going to have to be somewhere outside the airflow of your heater. As you might imagine, the best way to deal with this is always going to be to install the heater before you install the thermostat.