DSR requires at this time to have a GTX 980/970, and the latest drivers.
First open up the NVIDIA control panel and enable "DSR - Factors", Choose a DSR setting of your choice.
Next open up GeForce Experience, and select the game you want to enable DSR on, and click the wrench icon.
Select the available DSR resolution and click apply
GeForce Experience will now show the current resolution as a DSR resolution
Next confirm the resolution in game
With DSR enabled, you can also change the desktop resolution to match the DSR resolution within the NVIDIA control panel and Windows display settings, but image quality will not be clear. Games will also be able to detect the DSR resolutions within them and you can change the resolution in game as well.