Guillermo Luijk has a great page where you can find instructions on how to set up UniWB[a] on your camera, along with settings files. However, Nikon's D3200 will not actually compute a new white balance when you set the PRE white balance from a file. It just copies the WB setting from the file. In order to compute a new WB setting, you must use the Preset Manual / Measure option. So after determining which color your camera sees as neutral gray, take a defocused photo of it.