You mean preset white balance? Go to your camera menu, then shooting menu, select white balance, scroll to Preset manual, measure, it'll then ask Overwrite existing preset data, press yes & follow instruction on screen. Aim your lens a few inches away from the white paper & press the shutter release & your camera will capture & record the white balance & if done correctly, you'll see a msg that says data aquired. Then leave your white balance as preset. You got to do it in the place you'll be taking the photo.
If you move outdoors or move to another room with a different lighting from the place you recorded the white balance, you'll get the wrong white balance reading. So you got to do manual white balance everytime for different kind of lighting conditions or use the different set of white balance in the menu for different lighting condition. If not, just shoot in raw & adjust the white balance during pp.