Get a dremel and cut a hole for a fan. Easy as that.
Seriously, do you know what you system temps are now? You may not need to do that if you don't have problems with too much heat. If you still want to do it you'll want to make sure that you have about the same amount of air coming into the case compared to what your pushing out so you don't have a bunch of hot air circling around inside. I have two 80mm intake fans and two 80mm exhaust fans and that keeps my temps pretty cool.
I don't understand why people get thermometers to check their temps. There's a good program that reads the CPU and system temps from your motherboard and displays them in your taskbar, and it's probably more accurate than a thermometer and you won't have more crap inside the case. The program is called
Motherboard Monitor. It's very easy to set it up.