a network controller is typically a network card service or basically a driver for the network card.
typically it should be installed at the time when you install the hardware and install the drivers for the said hardware.
If you're having trouble locating drivers for your new network hardware, try googling for manufacturer/vendor name of your product followed by support word
majority of today hardware have dedicated websites that host the drivers
so, I guess to answer your original question, you install network controller by installing the drivers for it or if you don't have a network card you need to purchase one. You do not need to be connected to the internet in order for network controller to be installed