To get the most accurate colours, position a piece of white or neutral grey card or paper so that it is in the same lighting as your subject. For the best results, place the card where your subject will be, and make sure that it is facing where you are going to be shooting from.
Now fill the frame with the card and take a picture of it from the same direction as you will be taking your main shot. The exposure of this reference image isn’t critical, as long as the picture isn’t completely over- or under-exposed. You may need to switch to manual focus.
Now you need to navigate to your camera’s shooting menu and find the list of white balance settings. To do this on a Nikon, scroll down to the white balance settings and then select Preset Manual; on a Canon, simply select Custom White Balance from the list of options that appears.
Select the picture of the card to set this as the white balance reference image. On a Nikon, press OK twice to set the white balance, while on a Canon you need to first press Set to select the image, then go back to the white balance settings and select Custom from the list.