Download and install the most recent version of nUI if you have not already as you would any other addon. You should unzip the contents of the download file into your Interface\AddOns directory
Download and install this skin as you would any other addon and unzip it into the Interface\AddOns directory as well.
After downloading and installing both mods, you should have both an Interface\AddOns\nUI directory and an Interface\AddOns\nUI_Spiel directory
Run WoW and log in. Before selecting a character to play, click on the "AddOns" button in the lower left corner of the screen and enable the nUI Spiel addon. You can return to using the default nUI dashboard at any time by disabling nUI Spiel.
Select your character and enter the world. nUI will automatically replace the default dashboard with the nUI Spiel skin if it is enabled.