Everybody knows the importance of accurate white balance, and we all know that a simple and effective way to ensure accurate white balance is to shoot a gray card. But just in case you don't know exactly what that means, or how to go about doing it, here it is: a breakdown of how to use a gray card for perfect white balance every time.
1. Get the card. I carry a Kodak neutral gray card that I picked up long before I started shooting digitally. This card is handy for fast-paced shoots on the go because it's small and easily portable. When I've got a little more time, I carry a larger Macbeth color checker chart. Not only does it include various shades of gray, it also includes other colors, so I can determine—should I care to—how subtle light and white balance changes affect the actual colors in the scene. Some folks carry QP Cards—which I've also done, as they're super-portable and pocket friendly—or even three-dimensional cubes that help to show the shape of the light in question, along with its color. The most important thing is that the gray card be a neutral gray. So it's not enough to just paint a card gray, or find a "close enough" gray card from any number of makeshift sources. For this one, you really want to ensure that the neutral gray is truly neutral. Because that's how the white balance adjustment is made: gray should look gray, and your camera (or computer) will make color adjustments until that neutral gray actually looks neutral.
2. Shoot the card. Place your card in your scene and snap a picture. Simple enough, right? Well, true, but there's a catch. If you're shooting in a mixed light situation, you'll need to ensure you position your gray card so that all the various mixed light sources factor into the card accurately. Meaning if you position the card so that it is shaded from window light, it might only register the tungsten from above. The reverse is true too, all depending on where you put that card. And heaven forbid your card has a slight sheen or glossy coating to it, you'll have to be extra careful to ensure your card is not reflecting light into the lens and creating an unreadable highlight on the card where later you hope to find neutral gray. A good rule of thumb is to position the card near to the center of interest. In a portrait, that's usually the face. You can have your subject hold your card under their chin for a shot, and then you're all set. If you reposition your setup, you'll want to reshoot the card as well. That way you can have the most accurate white balance for the actual scene that ends up in your camera—as opposed to the one you initially planned and set up to shoot.
3. Click the card. With a shot of your gray card now in hand, you've got a variety of options for your next step. (With most D-SLRs, if you've shot a full-frame neutral gray image, you can set the in-camera custom white balance to base its settings off of this gray card shot. But that only works if you fill the frame with the gray card, and if you're not going to change lighting for the remainder of your shoot.) The method I prefer is to keep plugging along in a White Balance preset, occasionally reshooting the gray card as the lighting or location changes. Then I take that picture into the computer (when I import all of my images from a shoot) and use it to select a custom white balance after capture. I do this most effectively in Lightroom because I shoot RAW. No matter what RAW processing program you use—even Adobe Camera Raw—there's some variation on a custom white balance setting that provides you with an eyedropper to click in the neutral gray area of an image. Click the gray card and that sets the new custom white balance in the shot. Then it's just a matter of transferring that white balance setting to all the subsequent shots in the series. This works differently in different programs, but the principle is the same: make one perfect white balance setting and transfer it to multiple files. It's quick, simple and very effective. It's how you put all that gray card work into effect. One more thing: lest you think you have to shoot RAW to make a custom white balance change work, you can do the exact same thing to your JPEGs. It's still a great way to apply custom white balances to your images after you shoot them. The best approach for a JPEG shooter would be to change the in-camera custom White Balance setting for each shoot, but it's good to know you've got options once you get your pictures in the computer—as long as you made that simple little shot of that simple little card.