If you are using an existing GPU and have already installed Windows then download driversweeper and be sure to remove all existing display drivers properly after removing them with the control panel or better still, Revo Uninstaller.
Shut down the PC DO NOT reboot it, Windows will put in a display driver.
Ground yourself, side off the case, fit the card, plug in the PCIE power cables. Reboot the PC and install the latest driver from Nvidia. 260.99 or something IIRC.
Install a game, get a nice cold can of coke or whatever your fave beverage, enjoy.