There are several makers of white balance cards, Brian. I use the Lastolite Ezy Balance card which is sort of a 'plastic cloth' in a spring steel frame which folds up to pocket size and fits inside a zipped pouch when not in use.
Although, as Dan said, you can easily make adjustments when using Raw; but starting with a custom white balance should automatically put you very close to the ideal setting. Then just a few small tweaks to suit your own tastes.
All you do is place the 'card' at a suitable position and take a shot of it under normal average conditions. Then in the camera menu, select Custom White Balance and follow the simple instructions. That one setting will remain with you until you cancel it, or take another reading because the light has changed, etc.
Gradient Masks can be effective for the correct sort of scene, such as landscapes/seascapes, and similar situations where you have a large block area right across the image which requires adjustment; such as the sky.
For more complex areas, adding a mask then gently editing the mask to selectively change the opacity gives a much greater level of control which only effects certain required areas. Just brush over the area to be changed with a fairly soft brush set to a reasonably low opacity. Say somewhere around 20% so you gradually build up the desired result with several 'passes' of the brush. Don't attempt to do too much in one go because that can sometimes leave obvious 'edges' where the changes have started or stopped.
Dodging and Burning using masks is a powerful technique but it is something which has to be gradually learned. And one of the first big hurdles is deciding whether to use a 'hide or reveal' mask, then remembering which 'colour' of brush to use to add or subtract from the mask. This can initially appear somewhat complicated but it only requires a little bit of thought from a clear head and everything will become obvious.
Although, sometimes when I'm not thinking too clearly I still find myself painting away over a mask and wondering why nothing is happening - because everybody occasionally finds themselves working with the wrong type of brush or the wrong type of mask.