Download the drivers from somewhere. Put them on a pendrive, and connect it to the computer.
From there you can work with the terminal (actually, you could do everything from the terminal, I just don't know how to download stuff from it ). Either copy the file from the pendrive, or execute it from it.
You'll need to stop your Xorg server first. For Gnome, it should be something like /etc/init.d/gdm stop, then you'll have only the terminal, then sh the installer and follow the instructions. If something is missing it will probably fail and you'll just have to do whatever you have to do and sh the installer again. One of the common issues is that you need the linux kernel headers for your current kernel version and you need to have the exact GCC version installed that was used to compile the kernel.
To bring the Xorg server up again, its just /etc/init.d/gdm start.
You'll be better reading a guide in internet though, since I'm skimming over things.
Installing official ones through their website? All I kept reading is that nvidia didn't have very well supported drivers Linux at all and people have big issues with the latest ones.