Here's how it works: You shoot a picture of a blank piece of paper, the idea being that only the dust spots will show up in the resulting image. You then load that image into Nikon Capture NX-D, a free Nikon programs. From the reference image, the program creates a file noting the position of the dust spots. When you open a photo in Capture NX-D, you enable the program's Image Dust Off tool, which then applies some corrective magic just to the dust spots, consulting the reference file to know where those spots are located.
Unfortunately, the tool works only on photos that you shoot in the Raw format. In fact, you apply the Image Dust Off tool as you process the Raw file in Capture NX-D. Also bear in mind that the tool isn't always successful, sometimes resulting in a blurred area that's more problematic than the original dust spot. There's also a chance that the dust on the sensor may move slightly between photo shoots, making the reference photo inaccurate.
For specifics on capturing the reference photo, check the camera manual, available for download from the Nikon website.