Use the white card to set the custom white balance on the camera. You can use
Warm Cards instead if you want a warmer color balance.
Now having set the white balance, take a photo of the gray card (fully filling the frame with it) at what you feel is a good expousre setting. Look at the histogram of the photo. There should be a spike in the middle and not much else anywhere on the histogram. If its not in the center, adjust the exposure and shoot again till it is near the center. You now have the correct white balance and the correct expousre for the current lighting situation.
Here's a colorful photo to brighten up this discussion:
Cheers,
rfs